In triangle ABC, angle C is 90 degrees BC = 12, AB = 15. Find: cosA.

Given: right-angled triangle ABC;
angle C = 90 degrees;
BC = 12;
AB = 15.
Find cos A -?
Decision:
1) Consider a right-angled triangle ABC. By the Pythagorean theorem:
AC ^ 2 = AB ^ 2 – BC ^ 2;
AC ^ 2 = 15 ^ 2 – 12 ^ 2;
AC ^ 2 = 225 – 144;
AC ^ 2 = 81;
AC = 9 centimeters;
2) The cosine of an angle in a right-angled triangle is equal to the ratio of the adjacent leg to the hypotenuse. Consequently:
cos A = AC / AB;
cos A = 9/15;
cos A = 0.6.
Answer: cos A = 0.6.