2.4 Formulas
Business Formulas
Retail price = cost + markup
r=c+m
Profit = revenue – cost
p=r–c
Interest = Principal • rate • time
I = Prt
Ex: A student deposited $5,000 in a savings account. After one year, she received a check for
$137.50 in interest. Find the annual interest rate her money earned that year.
I = 137.5
P = 5,000
I = Prt
137.50 = 5000(r)(1)
137.50 = 5000r
5000
5000
0.0275 = r
r = 2.75%
r=?
t=1
Science Formulas
Distance = rate • time
d = rt
Fahrenheit to Celsius
5
C  ( F  32)
9
Ex: The minimum temperature on Mars is about -140⁰C. Convert the temperature to degrees
Fahrenheit.
C = 140
F=?
5
C  ( F  32)
9
5
 140  ( F  32)
9
9
9 5
(140)   ( F  32)
5
5 9
 252  F  32
 252  32  F  32  32
 220  F
F = -220⁰ F
Geometry Formulas
Perimeter formulas:
P = 2l + 2w
P = 4s
P=a+b+c
(rectangle)
(square)
(triangle)
Area formulas:
A = lw
A = s2
A = ½bh
A = ½h(B + b)
(rectangle)
(square)
(triangle)
(trapezoid)
Circle formulas:
D = 2r
r = ½D
C = 2πr = πD
A = πr2
(diameter)
(radius)
(circumference)
(area)
d
r
Ex: The circumference of the Earth is about 25,000 miles. Find its diameter to the nearest mile.
C = 25,000
C = 2πr = πD
C = πD
25,000 = πD
25,000 = πD
π
π
7,957.75 = D
D ≈ 7,958 miles
d=?
Volume formulas:
V = lwh
(rectangular solid)
3
V=s
(cube)
V = 4/3 π r3 (sphere)
See page 134
Solving Formulas for a Variable
Ex: Solve A = LW for L.
A = LW
A = L∙W
A = L∙W
W W
A
L
W
A
L
W
Ex: Solve x – y = -3 for y.
x – y = -3
x – y – x = -3 – x
-y = -3 – x
-1y = -3 – x
-1 -1 -1
y=3+x
y=x+3
Ex: Solve y = mx + b for m.
y = mx + b
y – b = mx + b – b
y – b = mx
y – b = mx
x
x
yb
m
x
yb
m
x
Application Problems
Ex: The cord of wood shown occupies a volume of 150 ft3. How long is the stack?
Ex: Use the fact that the fish is 10 inches long to find the area of the barbecue grill to the nearest
square inch.