Вычислить sin2 альфа: Sin альфа + Cos альфа=1/2

sin(2a) =?

sin(a) + cos(a) = 1/2;

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

(sin(a))^2 + 2 * sin(a) * cos(a) + (cos(a))^2 = 1/4;

(sin(a))^2 + (cos(a))^2 + 2 * sin(a) * cos(a) = 1/4;

1 + 2 * sin(a) * cos(a) = 1/4;

1 + sin(2a) = 1/4;

sin(2a) = 1/4 - 1;

sin(2a) = -3/4.

Ответ: -3/4.