In triangle ABC, angle C is 90 degrees, AB = 35, BC = 28. Find cos A.

The ABC triangle is rectangular.
1) AB = 35;
BC = 28.
Find cos a.
2) cos a = ac / ab;
3) Find the leg of a right-angled triangle by the Pythagorean theorem.
ac = √ (ab ^ 2 – bc ^ 2) = √ (35 ^ 2 – 28 ^ 2) = √ ((35 – 28) * (35 + 28)) = √ (7 * 63) = √ (7 * 7 * 9) = √ (7 ^ 2 * 3 ^ 2) = 7 * 3 = 21;
4) Calculate cos a.
cos a = 21/35 = 3/5 = 0.6.