### Home > PC > Chapter 7 > Lesson 7.3.2 > Problem 7-113

7-113.

Draw a reference triangle in Quadrant 1 for both angles.

Label the 3rd side.

Both are pythagorean triples: 3,4,5 and 5,12,13

Find sin *A* from the triangle sides.

Find cos *B* from the triangle sides.

Use the sum and difference formula:

sin(*A* + *B*) = sin*A*cos*B* + cos*A*sin*B*

Use the sum and difference formula:

cos(*A* + *B*) = cos*A*cos*B* − sin*A*sin*B*