In a triangle, the angle is 90 °, the sine of the outer angle at the vertex A is 7/25. Find cosA.

As you know from the school curriculum of mathematics, the outer and inner angles together make up the value of 180 °.

At the same time, based on one of the basic trigonometric formulas, namely sin (180 – a) = sin a, we can conclude that the sine of angle A will also be equal to 7/25, like the sine of the outer angle.

Determine what the cosine of angle A will equal using another trigonometric formula:

cos2 A = 1 – sin2 A;

cos2 A = 576/625;

cos1 A = 24/25

cos2 A = – 24/25 (not suitable).

Answer: 24/25.