Question

How do you use the law of cosines or law of sines if you are given a=11, b=7, c=9?

Answer 1

To find a triangle's angle use the Law of Cosines

a

2

=

b

2

+

c

2

−

2

b

c

.

cos

A

11

2

=

7

2

+

9

2

−

2

(

7

)

(

9

)

cos

A

cos

A

=

130

−

121

126

=

9

126

=

0.07

--> A = 85.90

b

2

=

a

2

+

c

2

−

2

a

c

.

cos

B

49 = 121 + 81 - 198cos B

cos

B

=

153

198

=

0.77

-> B = 39.40

Angle C = 180 - B - A = 180 - 39.40 - 85.90 = 54.70