[tex]\bf tan(\theta)=\cfrac{sin(\theta)}{cos(\theta)}\qquad sec(\theta)=\cfrac{1}{cos(\theta)}
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sin^2(\theta)+cos^2(\theta)=1\implies cos^2(\theta)=1-sin^2(\theta)\\\\
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%sec x tan x(1-sin^2x)=sinx
sec(x)tan(x)[1-sin^2(x)]=sin(x)\\\\
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sec(x)tan(x)[1-sin^2(x)]\implies \cfrac{1}{\underline{cos(x)}}\cdot \cfrac{sin(x)}{\underline{cos(x)}}\cdot \underline{cos^2(x)}\implies sin(x)[/tex]
