# math (trigonometry)

if tan A/2 =cosecA-sin A then prove cos^2
A/2=cos 36 degree

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1. tan A/2 = (1-cosA)/sinA

So, you have

(1-cosA)/sinA = 1/sinA-sinA = (1-sin^2A)/sinA = cos^2A/sinA
so
cos^2A+cosA-1 = 0

Solve that for cosA, and then use the fact that

cos36° = (1+√5)/4

to prove the result

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2. That is negetive and posetive

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