Sunday, August 9, 2015

Prove the identity: cos(theta) / 1 - sin(theta) = sec(theta) + tan(theta)

The identity to be proved is: cos(theta) / 1 - sin(theta)
= sec(theta) + tan(theta)


I'll rewrite this with x used
instead of theta


We have to prove: cos x/ (1 - sin x) = sec
x + tan x


Let's start from the right hand
side


sec x + tan x


substitute
sec x = 1/cos x and tan x = sin x / cos x


=> 1/ cos
x + sin x / cos x


=> (1 + sin x)/cos
x


multiply the numerator and denominator by (1 - sin
x)


=> (1 + sin x)(1 - sin x)/(cos x)*(1 - sin
x)


=> (1 - (sin x)^2)/ (cos x)*(1 - sin
x)


=> (cos x)^2 / (cos x)*(1 - sin
x)


=> cos x / (1 - sin
x)


which is the left hand
side


This proves the identity cos(theta)/(1 -
sin(theta)) = sec(theta) + tan(theta)

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