Question

Please help me. How can I find 10 years ago

Answer 1

use the cosine rule

Answer 2

use cos B=(a*a+c*c-b*b)/2ac

Answer 3

Hint:Use the laws of cosine.

Answer 4

place the side opposite angle B on left side of the equation
10^2 = 12^2 + 15^2 - 2*12*15 cos B

Answer 5

solve for cos B

Answer 6

where side a is opposite to angle A and etc.