Вычислим cos (a/2), если cos a = -7/25.

cos a = cos^2 (a/2) - sin^2 (a/2) = cos^2 (a/2) - (1 - cos^2 (a/2)) = cos^2 (a/2) - 1 + cos^2 (a/2) = 2 * cos^2 (a/2) - 1;

cos a = 2 * cos^2 (a/2) - 1;

-7/25 = 2 * cos^2 (a/2) - 1;

-7/25 + 25/25 = 2 * cos^2 (a/2);

2 * cos^2 (a/2) = 18/25;

cos^2 (a/2) = 9/25;

cos (a/2) = +-3/5.