How to use the sine law and cosine law to solve a triangle problem?

Question:

In the figure shown, ABC is a triangle. BA = BD = a (a is a number.) AB = (square root of 2) × AD, and 3 × AB = (square root of 2) × AC. Find the degree measure of the angle C.

Solution:

Because we know the three sides of the triangle ABD and we also know the side AC which opposite the angle B in triangle ABC, we need to find the angle B first.

cos B = BA² + BD² - AD² ÷ [2 × BA × BD]

we are given:

AB = BD = a

AB = (square root of 2) AD, so, AD = AB/(square root of 2) = a/(square root of 2)

3 AB = (square root of 2) AC, so, AC = 3 AB/(square root of 2) = 3a/(square root of 2)

cos B = [a² + a² - (a/(square root of 2))²] / [2 × a × a]

= [a² + a² - a²/2] / 2a²

= a²(1 + 1 - 1/2)/2a²

= (2 - 1/2)/2

= 3/4

find the value of sin B

sin²B = 1 - cos²B

= 1 - (3/4)²

= 1 - 9/16

= 7/16

sin B = (square root of 7)/4

In triangle ABC, we know the angle B and its opposite side AC, and we also know the side AB which opposite the angle C. Use the sine law to find the angle C.

In triangle ABC,

sin C/AB = sin B/AC

sin C = (AB/AC) sin B

= a/[3a/(square root of 2)] × (square root of 7)/4

= (square root of 2)/3 × (square root of 7)/4

= (square root of 14)/12

= 0.3118

angle C = arc sin 0.3118 = 18.17^o

Therefore, the angle C is 18.17 degrees, please watch the video for more details.