1a MATH 31 UNIT 1 –TANGENTS AND LIMITS Tangents to a Curve Slope = m = rise y y2 y1 f ( x2 ) f ( x1 ) run x x2 x1 x2 x1 If we use a variable point P(x,y) and the slope, m, is known the equation of the line comes from the y y1 point–slope formula: m x x1 For parallel lines m 1 = m 2; 1. for perpendicular lines m 1 m 2 = –1 or m1 What is the equation of a line perpendicular to y = 2x + 4 if this perpendicular line is on (6, 2)? 1 1 m2 1b 2. What is the equation of a line parallel to 4x + 2y – 7 = 0 and having an x–intercept at –3. Equation of a Tangent Line to a Curve For a curve the slope of the tangent to the curve is taken as the limiting value of the slopes of a sequence of secant lines for which the point of tangency is fixed as one end of all the secant lines. Tangent – touches but does not cut a curve Secant – Cuts a curve in 2 or more points Chord – Line segment between secant line intercepts 2 1c 1. What is the equation of the tangent to y = x 2 at the point P(2, 4)? We could use either side but we will find the slope of PQ n as Q n approaches P from the left. (As n takes values 1, 2, 3,...the point Q n approaches P) 5 Y P(2,4) Q(1.5, 2.25) Q(1,1) X Q(0,0) 0 P(2, 4) Slope of PQ n = m Qn y2 y1 f ( x2 ) f ( x1 ) = x2 x1 x2 x1 -5 4 0 22 ANS 2 Grapher - http:/ / www.alentum.com/ agrapher/ 2 Q 1 (0, Created 0) with a trial version of Advanced Slope of PQ 1= 20 2 ANS Q 2 (1, 1) Slope of PQ 2 = 4 1 3 2 1 Q 3 (1.5, 2.25) Slope of PQ 3 = 4 2.25 1.75 3.5 2 1.5 .5 Q 4 (1.9, 3.61) Slope of PQ 4 = 4 3.61 .39 3.9 2 1.9 .1 Q 5 (1.99, 3.9601) Slope of PQ 5 = 4 3.9601 .0399 3.99 2 1.99 .01 Q 6 (1.999, 3.996001) Slope of PQ 6 = 3 .003996001 3.999 .001 1d As Q n approaches P, the slope of the secant line PQ n is getting closer and closer to being the same as the slope of the tangent at P. In this case the slope seems to be approaching 4 so we can say that the slope of the tangent to y = x 2 at P(2, 4) is m tangent = 4. What is the equation of the tangent at P(2, 4) on y = x 2 ? m y y1 x x1 4 y4 x2 4x – 8 = y – 4 y = 4x – 4 * If we had used Q n values of 5, 4, 3, 2.5, 2.1, 2.01, 2.001 etc the slope of the tangent would still be 4 but we would have approached P from the right (from above 4) Read p(5 – 9) Do p9 #1, 2, 4, 7, 9 4 2a Limits A limit is a value that an expression is approaching. By lim f ( x) L we mean that as x x a gets closer and closer to having a value of a, f(x) gets closer and closer to having a value of L. The limit is said to equal L even if x will never be allowed to equal a. 1. lim x 5 x x2 x 4 4.5 4.9 4.999 4.99999 6 5.5 5.1 5.001 5.00001 y x y Simple limit can be found by substitution as in lim f ( x) f (a) . x a 2. lim x 7 3. lim x 2 7 4. 5x 3 lim x x 1 x 3 x 5 5 2b 5. x2 4 lim x 2 x2 For more complicated limits we need to understand the properties of limits. Given c is a constant and lim f ( x ) and lim g ( x) both exist. x a xa 1. lim f ( x) g ( x) lim f ( x) lim g ( x) 2. lim f ( x) g ( x) lim f ( x) lim g ( x) 3. lim f ( x) g ( x) lim f ( x) lim g ( x) x a x a xa 4. f ( x) f ( x) lim x a lim x a g ( x) g ( x) lim x a 5. lim cf ( x) c lim f ( x) 6. lim f ( x) lim f ( x) x a x a x a x a x a x a x a xa n xa x a n for n a positive integer or fraction Simple limit can be found by substitution as in lim f ( x) f (a) . x a If f(a) does not exist (causes division by zero) a limit may still exist but it will be found only after some trick of factoring and simplifying, or rationalizing, to make finding the limit possible. 6 2c Find the limits: 1. x2 7 x 2 7 lim x 5 lim x 5 x 2 x 2 lim x 5 2. 3x 2 8 x 7 lim x 4 2x 5 3. 1 lim x x 4. 5. 1 lim x 0 x 6. 7. x 2 49 lim x 7 x7 8. x3 27 lim x 3 x 3 9. 5 h 2 52 lim h 0 h 7 1 lim 1 x x7 1 lim x 3 x 3 2d 10. 11. x 9 lim x 9 x 3 x 125 lim 3 x 125 x 5 Read p(11 – 18) Do p(18 – 20): #1, 8, (2 – 6) a, c, e 8 3a One Sided Limits 10 x2 4 For limits such as lim we found that x 2 x 2 Y 5 x 2 x 2 x 4 lim lim x 2 2 2 4 lim x 2 x2 x 2 x 2 x 2 2 This can also be seen from the graph of y= -10 0 -5 5 10 x 2 4 ( x 2)( x 2) x 2, x 2 x2 ( x 2) -5 We see that whether x approaches 2 from the left (x < 2) or x approaches 2 from the right (x > 2); the limit is still 4. -10 Created with a trial version of Advanced Grapher - http:/ / www.alentum.com/ x 4 x2 4 and lim 4 lim 4. x 2 x 2 x2 x2 2 We can write this as Since If x2 4 x2 4 lim 4 lim x 2 x 2 x2 x2 x2 4 we can say lim 4. x 2 x 2 lim f ( x) L lim f ( x) then lim f ( x) L x a but if x a x a lim f ( x) lim f ( x) then lim f ( x) Does not exist x a x a x a 9 3b 10 1. 2 x if x 1 Consider the function: f ( x) x 1 if x 1 c) 5 lim f ( x) a) b) Y x 1 lim f ( x) X x 1 -10 0 -5 5 10 lim f ( x) x 1 -5 Y 5 2. Some functions only have limits from 1 side. -10 Created with a trial version of Advanced Grapher - http:/ / www.alentum.com/ agr for f(x) = x 1 lim f ( x) lim x 1 x 1 x 1 X -10 -5 0 5 10 lim f ( x) lim x 1 x 1 x 1 Thus lim x 1 x 1 Created with a trial version of Advanced Grapher - http:/ / www.alentum.com/ agrapher/ A function may be continuous if there are no breaks in the graph ( y = x, y = x 2, y = sinx) A discontinuous function is one that has breaks or holes in it. ( # 1 was discontinuous at x = 1, y= x2 4 1 is discontinuous at x = 4, y = is discontinuous at x = 2.) x4 x2 10 3c A function is continuous at x = a if lim f ( x) f (a) . x a For this to happen: 1. f(a) must be defined 2. lim f ( x) must exist. i.e. lim f ( x) lim f ( x) xa x a x a 3. lim f ( x) f (a ) xa 10 Y y = f(x) Which of the 3 conditions fail at each of the following discontinuities? 5 1. 1 f(x) = x 3 X -10 -5 0 5 10 -5 -10 Created with a trial version of Advanced Grapher - http:/ / www.alentum.com/ agrapher/ 11 10 2. f(x) = x2 4 x2 3d Y 5 X -10 0 -5 5 10 -5 -10 Created with a trial version of Advanced Grapher - http:/ / www.alentum.com/ agrapher/ . Y 3. 10 3 for x 2 and x 2 f(x) = 1 for x 2 y = f(x) 5 X -10 -5 0 5 10 -5 -10 Created with a trial version of Advanced Grapher - http:/ / www.alentum.com/ agrapher/ 12 10 Y y = f(x) 3e eg Given the function y = f(x) 1. f(2) = 2. f(–2) = 3. f(3) = 4. 5. 6. 7. 8. 5 X -10 -5 0 5 10 -5 lim f ( x) x 2 lim f ( x) x 2 -10 lim f ( x) x 2 Created with a trial version of Advanced Grapher - http:/ / www.alentum.com/ agrapher/ lim f ( x) x 2 lim f ( x) x 2 9. lim f ( x) 10. There are discontinuities at x = –2, 2 and 4 x2 Read p(21 – 26) Do p 27 # (1, 2, 4)( a, e, f, h), 3, 7, 10(a,b), 12 13 4a Using Limits to Find Tangents For a function such as y = x2 we can find the limiting value of the slopes of a sequence of secants mPQn and use it to Y approximate the slope of the tangent at P. i.e. As Q n changes from Q 1 to Q 2 to Q 3 etc. and ‘n’ increases as a natural number as Q n gets closer and closer to P P(x,y) Q4 Q1 Q3 Q2 the slope of the secant PQ mPQn will get closer and closer to the slope of the tangent at P. m tan gent at P lim m sec ant PQn Qn P Created with a trial version of Advanced Grapher - http:/ / www.alentu This idea can be used to find the slope of the tangent to the curve at: 1. A fixed point eg. (3,f(3)) 2. A generic point eg. (a,f(a)) 3. A variable point eg. (x,f(x)) The process is essentially the same but it can be done using 2 different formulas: 14 4b eg. f ( x) f (a) y lim x 0 x x a x a Formula 1 m lim Formula 2 m lim f ( a h) f ( a ) y f ( x h) f ( x ) lim or m lim x 0 x h 0 h 0 h a h a Use Formula 1 and 2 to find the slope of the tangent to f(x) = x2 at the point A(3,9) Formula 1 f ( x) f (a) y Q2 lim x 0 x x a x a Y m lim B(x,f(x)) slope of tangent at A(3,9) = f(x) - f(3) A(3,9) x-3 Created with a trial version of Advanced Grapher - http:/ / www.alentum.com 15 4c Formula 2 f ( x h) f ( x ) y lim x 0 x h 0 x h x m lim Y Q2 slope of tangent at A(3,9) = B(3+h,f(3+h)) f(3+h) - f(3) A(3,9) (3+h) - 3 = h X Created with a trial version of Advanced Grapher - http:/ / www.alentum.com/ agraph eg. Use Formula 1 to find the slope of the tangent to f(x) = 3x2 + 5 at A(2,17) slope of tangent at A(2,17) = lim x 0 y f ( x) f (2) lim x 2 x x3 16 4d eg Slope of tangent to y = x3 5x + 5 at x = 2 10 9 8 7 6 5 4 3 2 1 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1-10 -2 -3 -4 -5 -6 -7 -8 -9 -10 Read p (30 – 35) Do p35 # 1, 2, 6, 8 Y 1 2 3 4 5 6 7 Created with an unregistered version of Advanced Grapher - http 17 4e Slopes of Tangents at any Point To avoid separate calculation when we want the slope of the tangent to a curve at more than 1 point we can obtain an expression for the slope of the tangent at a generic point (a, f(a)) and then find specific slopes by substituting specific values of ‘a’ into this slope expression. eg Use Formula 1 and the point (a, f(a)) to obtain an expression for the slope of the tangent to f(x) = 2x2 – 4x + 1, then find the slopes of the tangents to this function at x = 0, 1 and 4. Formula 1 f ( x) f (a) y lim x 0 x x a x a mtan gent lim at x = 0 at x = 1 at x = 4 18 4f Where on this curve (f(x) = 2x2 – 4x + 1) would the tangent have a slope of 24? eg Use Formula 2 and the point (a, f(a)) to obtain an expression for the slope of the tangent 3 to y at the point (a, f(a)); then find the equations of the tangents to this function x2 at x = –5 and x = 0. Formula 2 f ( x h) f ( x ) y lim x 0 x h 0 h mtan gent lim 19 eg Where on the curve y 3 3 which has a slope at x = a of mtan gent is the slope x2 (a 2) 2 3 ? 64 eg. What is the equation of the tangent line to y = x at the point (a, a )using Formula 1. What are the equations of the specific tangent lines at (4, 4 ) or (4, 2) , (9,3) and (49, 7)? 20 x have a slope of 4? eg Where does the tangent line to y = eg. Using the secant slope expressions what are the slopes of the secants from 4 to 9; 4 to 49? Do p35: # 7(iv, v. vi), 9, 10 21 5a Velocity, Rates of Change On a graph of displacement, s, as a function of time, t, or s = f(t), the slope of the secant line, m = Ds m , would have units of and Dt s would represent the average velocity, vavg , over the time interval, D t . Y Q2 vavg = st - st1 Ds = 2 Dt t 2 - t1 Q(t+dt,s+ds) delta s P(t,s) If we take the points P(t,s), and Q (t + D t , s + D s ). then vavg = st 2 - st1 t 2 - t1 = delta t X (s + D s ) - s D s = Dt (t + D t ) - t As t approaches 0, Q approaches P and the secant line PQ becomes a tangent line at P. The slope of the tangent at P(t,s) would be the limiting value of the slopes of the Created with a trial version of Advanced Grapher - http:/ / www.alentum.com/ agraphe sequence of secants; i.e. the limiting value of the average velocity over the time interval, t, as t 0. i.e. the slope of the tangent at P(t,s) will be the instantaneous velocity at P. vavg = Ds Dt but æD s ö vins t an t an eous = v = lim çç ÷ ÷ D t ® 0 èD t ÷ ø 22 5b eg. If g = 10 and the distance a body falls is given by s = 0.5(10)t2 = 5t2. a) What is the average velocity in the third second? ( third second is from t = 2 to t = 3) b) What is the instantaneous velocity at t = 3 s? Formula 1 f (t ) f (3) s lim t 0 t t 3 t 3 m lim 23 5c Formula 2 f (3 h) f (3) s lim t 0 t h 0 3 h 3 m lim Read p (36 – 43) Skip pages (45 – 57); Do p 43 # 1, 3, 7, 9 Do p 58 # 1, 2, 6, 8, 10, 3(f,g,h), 4(c,d,h) Additional Questions 1. Given y = x3 2x + 1, find: a. the slope of the tangent at any point. ans: m = 3a2 2 b. the point(s) where the slope will be 10. ans: (2, 5) and (2, 3) 24 5d c. the equation of the tangent at the point where x = 1. ans: x y + 3 = 0 d. use the secant slope expression to obtain the slope of the secant from x = –2 to x = 2 ans: m = 2 2. Rationalize the numerator in ans: x 5 x2 3 . (simplify) x 1 ( x 2) ( x 5 x 2 3) 3. Evaluate the following limits. 3 6 5 x b. lim x 10 x 10 lim x 6 2 a. x 6 ans: doesn't exist ans: 4. Find the slope of the tangent at x = 3 for y = ans: x . x4 4 49 25 3 50 5e 5. Find the slope at any point for y = ans: m 6. 3x 1 . 3 2 3x 1 10 9 8 7 6 5 4 3 2 1 4 x if x 0 Graph f ( x) x 2 1 if 0 x 2 and find: 8 x if x 2 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1-10 -2 -3 -4 -5 -6 -7 -8 -9 -10 Y X 1 2 3 4 5 6 7 8 9 10 a. lim f ( x ) ans: doesn't exist lim with f ( an x)unregistered b. Created ans: 5version of Advanced Grapher - http:/ / www.serpik.com/ agrapher/ c. ans: 6 d. lim f ( x ) x 0 e lim f ( x) x 2 f(0) = x2 x2 f(2) = ans: 2, does not exist f. 26 ans: doesn't exist Any Discontinuities? ans: x = 0, 2 6a Infinite Limits When we have functions that are undefined for a certain value of x, this expression has a behaviour bout that number. To evaluate this behaviour, we look at the limits. Vertical Asymptote vertical line that graph gets closer to as "y values" gets very big If 1. 2. lim f (x) or if lim f (x) then x = a is a vertical asymptote. x a lim x 0 lim x 0 x a 1 1 1 , lim lim does not exist , x 0 x 0 x x x 1 x 10 1 doesn ' t exist, lim x 0 x , lim x 0 1 x x = 0 is a vertical asymptote does not exist , x = 0 is a vert. asy. Y 8 3. lim x 2 Y 1 1 1 10 , lim , lim , 2 2 x 2 x 2 (x 2) (x6 2) (x 2) 2 Y 16 asymptote x = 2 is a vertical 8 12 4 6 8 2 4 X -10 -8 -6 -4 -2 0 2 4 6 8 4 10 2 -2 -4 X -10 -8 -6 -4 -2 0 2 -2 4-12 6 -10 -88 -610 -4 -2 0 2 4 6 8 10 12 -4 -6 -4 -8 -8 Determining Vertical Asymptotes -6 -10 determine when the denominator equals -8zero Created with a trial version of Advanced Grapher - http:/ / www.alentum.com/ agrapher/ -10 -12 find limit of a function from left and right, if either is or – then a V.A exists at this Created with a trial version of Advanced Grapher - http:/ / www.alentum.com/ agrapher/ Created with a trial version of Advanced Grapher - http:/ / www.alentum.com point 27 -16 6b 1. y 2 x 2x 8 2 2, Sketch f(x) = x 1 x2 9 intercepts at Assignment pg. 204 # 1 a, c, f, g, i, 2 pg. 206 # 1, 2 b, d, f, h pg. 212 # 1 a, b, 2 a, b, e, k, 3 a, b, d, e, f, g (draw graph near asymptote on # 3 d, f) 28 7a Horizontal Asymptotes the horizontal line that a graph gets closer and closer to but usually doesn’t touch as "x" gets very big (+ or –) lim f (x) a If x or if lim f (x) a x , then y = a is a horizontal asymptote. * A function could have two different horizontal asymptotes (one for x and one for x ) 1 0 x x r Note that for any positive rational number, r, lim eg lim 1 x x 1 4 0 1. Find the following limits. (* divide each term by the highest degree variable) 2x x x 3 a. lim b. 2x x x 3 c. lim d. 2 x3 x x 3 lim 29 3x 3 3x x 7x 3 2 lim 7b 2. Find the horizontal asymptotes. a. y 2x x 5 b. y x2 x3 2 3. Find the vertical and horizontal asymptotes, x & y-intercepts, and sketch the graph. a. y 2x 6 x5 x-int: possible vertical asymptote: horizontal asymptote: b. y y-int: x 2 3x x 2 7x 12 Assignment: pg.223 # 2 a, c, e, g, i, 3, 4 b, e, 5 a, d 30 8a Limits of Trig Expressions lim sin x 0 x 0 lim cos x 1 x 0 3 2 1 15 1 17 lim 5cos x sin x 5 2 2 4 2 4 x 6 2 lim x 0 sin x ? (1) x Proof: (not necessary): C B sin O A D OD = OC = 1 cos AsectorOBA AOCA AsectorOCD 1 1 1 (OA) 2 () (OA)(AC) (OD) 2 () 2 2 2 1 1 1 2 cos 2 () cos sin 12 () multiply by 2 2 2 cos sin 1 cos cos sin 1 sin sin lim cos l lim 1, so lim 1 0 0 0 cos 31 8b sin(3x) x 0 x eg 1. lim sin 5x x 0 8x 2. lim 3. lim 2sin x cos x x 4. lim cosh 1 h x 0 h 0 32 8c cosh 1 h 0 h 4*. lim sin 3x x 0 sin 4x 5. lim 1 cos 2x x 0 x2 6. lim 7. sin(3x) 5 x x lim 6 33 8d sin( 5x) x 0 2x 8. lim Assignment Read p 296 – 306 pg. 299 # 1 pg. 306 * Do not use calculator. # 1, 2, 4, 8, 12, 15, 18, 19, 28 34 9a Exponential Functions When looking at the limits of exponential functions, we need to evaluate the graphs of our basic exponential functions. Look at the graph of y = 2x 5 Y X 0 5 We notice that this graph as an asymptote at y = 0, so using this idea we know that lim 2 x = 0. If we are approaching -∞ along the x, we are approaching the asymptote of 0. x What is happening to the graph as x is getting bigger? lim 2 x . x Do all exponential graphs look this way? What if the base is between 0 and 1? 1 Look at the graph of y 2 x 35 5 Y X 0 5 This has different properties because it is reflected about the y – axis. Using this graph, we x x 1 1 can see that lim and lim 0 . These are the basic properties for the limits of x 2 x 2 exponential functions. Pay attention to the base of the exponential function. A negative exponent will reciprocate the base (reflect about the y – axis)! Ex. 1. lim(1.1) x 2. lim(0.3) x x x 1 3. lim (3.2) x 4. lim(3) x x x 5. lim(2) cos x x0 e) 7. lim( x 1 2 x 1 6. lim (5) x2 Assignment: Pg. 361 # 2, 3, 5 (all) 36 1 x2 10a Log Function y = logbx, b > 0, b 1 by = x inverse of y = bx (reflected over line y = x) y = 10x is the inverse of y = logx y = ex is the inverse of y = lnx = logex 2. eln Simplify: 1. ln e7 2 e 4 x 3. 2ln 5e + ln 2e3 ln 50e = x 4. 5. ln 3 + ln 8 = ln 2 + log x 6. eln e eln 1 37 32 10b When solving limits and graphing it is easier if you remember the basic graphs of y = ax and its inverse y = loga x for the 2 cases: Y Y a>1 0<a<1 y = log(x) X -10 -5 0 5 10 X -10 0 -5 5 10 y = log(x) Created with a trial version of Advanced Grapher - http:/ / www.alentum.com/ agrapher/ Created with a trial version of Advanced Grapher - http:/ / www.alentum.com/ agrapher/ 1. y = log2x domain: range: vertical asymptote: 10 9 8 7 6 5 4 3 2 1 (x = 2y) -10 -9 -8 -7 -6 -5 -4 -3 -2 -1-10 -2 -3 -4 -5 -6 -7 -8 -9 -10 Y X 1 2 3 4 5 6 7 8 9 10 Created with an unregistered version of Advanced Grapher - http:/ / www.serpik.com/ agrapher/ 38 10c 2. Find the domain for log2(x2 9) = y 3. Draw the graph of y = log3(x 2) 4. Find the following limits. y a. b. lim log 0.5 (x 2) 1 * 2 lim ln(cos x 1) * ey x 2 x 39 10d c. lim ln x * ey = x, x > 0, x 0 d. lim e tan x = x 2 e. lim e x 5 x 5 f. g. = lim ln x 3 = x 3 lim e x 2 1 cos x 2 5. Compare the graphs of y = log(x2) and y = 2 log(x). y = 2 log(x). has a vertical stretch by 2 which does not affect the range y R but the domain is x > 0 y = log(x2)has the same range y R, but the domain doubles ( x < 0 or x > 0) as the graph opens both ways since x2 is also positive for x < 0. 40 10e Assignment pg. 353 # 3 b, g, j, 4 c, f pg. 354 # 1 b, d, 2 a, b, 3 a, b pg. 375 # 2 a, c, d, 3 a, b, e, 4 a, c, 5 a, 41 6 a, 7 b, c 9 a, c
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