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In my text book there are three formulae:

cos^2ttheta + sin^2theta = 1

1 + tan^2theta = sec^2theta

cot^2theta + 1 = cosec^2theta

I am also stuck in this question:

prove this identity:

cosA / (1 - tanA) + sinA / (1 - cotA) = Sin A + cosA

Since this requires a proof I need to work on the LHS and RHS separately. Can someone help me start this question? I really appreciate your help.