Chứng minh rằng
a) sin(270ο - α) = -cosα;
b) cos(270ο - α) = -sinα;
c) sin(270ο + α) = -cosα;
d) cos(270ο + α) = sinα.
a) sin(270ο - α) = sin(360ο - (90ο + α)) = -sin(90ο + α) = -cosα
b) cos(270ο - α) = cos(360ο - (90ο + α)) = cos(90ο + α) = -sinα
c) sin(270ο + α) = sin(360ο - (90ο - α)) = -sin(90ο - α) = -cosα
d) cos(270ο + α) = cos(360ο - (90ο - α)) = cos(90ο - α) = sinα
-- Mod Toán 10