1.2 SOHCAHTOA of Obtuse Angles.notebook
1.2 SOH CAH TOA for Obtuse Angles
On graph paper draw the point P (6,8) in Quadrant 1. Construct right triangle PBA as shown. How can you determine the length of r?
y
10
9
8
7
6
5
4
3
2
1
­10
­8
­6
­4
­2
0
­2
­3
­4
­5
­6
­7
­8
­9
­10
(6 ,
8)
r
y
x
2
4
6
8
10
x
Using the side lengths as they are labelled, write the trig ratios for:
sin A =
cos A=
tanA=
1
1.2 SOHCAHTOA of Obtuse Angles.notebook
On the same grid, choose a second point in Quadrant II and label as shown below
y
10
9
8
7
6
5
4
3
2
1
(­6 , 8)
­10
­8
­6
­4
­2
0
­2
­3
­4
­5
­6
­7
­8
­9
­10
x
2
4
6
8
10
Using the side lengths as they are labelled, write the trig ratios for:
sin A =
cos A=
tanA=
What do you notice about the sign of some of the coordinates?
How does this affect the values of the trig ratios?
2
1.2 SOHCAHTOA of Obtuse Angles.notebook
Complete the table using the results you have just discovered:
Angle Measure sinA
cosA
tanA
Acute <A
Obtuse <A
3
1.2 SOHCAHTOA of Obtuse Angles.notebook
This leads us to a rule that will tell us which Trig Ratio is positive in each quadrant:
y
10
9
8
7
6
5
4
3
2
1
S
­10
­8
­6
T
­4
­2
0
­2
­3
­4
­5
­6
­7
­8
­9
­10
A
x
2
4
6
8
10
C
4