Answers

2014-12-18T18:41:16+08:00
\frac{1-cot ^{2}}{1+cot ^{2}} =sin ^{2} - cos^{2} 
 \frac{1- \frac{cos ^{2} }{sin ^{2} } }{csc ^{2} } =sin ^{2} - cos^{2} 
 \frac{sin ^{2}-cos ^{2} }{sin ^{2} } * sin^{2} =sin ^{2} - cos^{2} 
sin ^{2} - cos^{2} =sin ^{2} - cos^{2}

pythagorian,division and reciprocal identity

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