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2015-05-21T14:10:27+08:00

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CotAcosA=cscA-sinA
I prefer moving the left side of the equation, doing nothing to the right side
\frac{cosA}{sinA}(cosA)=cscA-sinA
Now multiply cosA by itself:
\frac{cos^2A}{sinA}=cscA-sinA
doing the conversion
cos²x+sin²x=1

\frac{1-sin^2A}{sinA}=cscA-sinA
separate the two
\frac{1}{sinA}-\frac{sin^2A}{sinA}=cscA-sinA
since:
\frac{1}{sinA}=cscA
and 
\frac{sin^2A}{sinA}=sinA
substitute:
cscA-sinA=cscA-sinA

Hope this helps =)
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