These are the cofunction identities. Please scrutinize carefully. s sin = cos(π/2 - 0) cos sin (π/2 - 0) tan 0 = cot(π/2 - 0) cot 0=tan(π/2 - 0) sec 0 = сsc(π/2 - 0) csc 0 sec(π/2 - 0) = O True