2. The relationship between complementary trigonometric functions.

sin(90 -α)=cosα，cos(90 -α)=sinα，

tan(90 -α)=cotα，cot(90 -α)=tanα。

Basic formula of trigonometric function

Sinθ= diagonal (sine),

Cosθ= hypotenuse of adjacent side (cosine),

Tanθ=sinθ cosθ (tangent)

Cotθ=cosθ sinθ (cotangent),

Secθ= 1 cosθ (secant),

Csθ= 1 sinθ (cotangent)