Given that,
ABC is a right angle triangle, C is the right angle.
To find,
cosB = ?
Solution,
The sum of angle of a triangle is equal to 180 degrees. So,
∠A + ∠B + ∠C = 180°
Put ∠C = 90°
∠A + ∠B + 90° = 180°
∠A + ∠B = 90°
Taking cos with each term.
cosA+cosB = cos (90)
cosA+cosB = 0
cosB=-sinA
Hence, cosB = sinA.