formula

(sinα)^2+(cosα)^2= 1

1+(tanα)^2=(secα)^2

1+(cotα)^2=(cscα)^2

To prove the following two formulas, just divide one formula by (sin α) 2 and the second formula by (cos α) 2.

(4) For any non-right triangle, there is always

tanA+tanB+tanC=tanAtanBtanC