What is cosπ-a equal to?

Cos(π-α)=-cosα. This is an inductive formula.

You can also use the sum angle formula: cos(α-β)= cosαcosβ+sinαsinβ to derive:

cos（π-α）= cosπcosα+sinπsinα=-cosα。

The relationship between π-α and the trigonometric function value of α can be obtained by using Equation 2 and Equation 3:

sin（π-α）= sinα。

cos（π-α）=-cosα。

tan（π-α）=-tanα。

Kurt (π-α) =-Kurt α.

Basic relations of trigonometric functions with the same angle;

Reciprocal relationship: tanα cotα= 1+0, sin α CSC α = 1, cosα secα= 1+0.

The relationship of quotient: sinα/cosα=tanα=secα/cscα, cosα/sinα=cotα=cscα/secα.

And the relationship: sin2α+cos2α= 1, 1+tan2α=sec2α, 1+cot2α=csc2α.

Square relation: sin? α+cos？ α= 1。