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[phuonglien99]

Biết $tanx,tany$ là nghiệm pt: $x^{2}+px+q=0$

                                                 

         CMR:  $\sin ^{2\left ( x+y \right )}+P\sin \left ( x+y \right )\cos \left ( x+y \right )+q\cos ^{2}\left ( x+y \right )$

Edited by phuonglien99, 04-02-2014 - 09:32.

[phuonglien99]

Bài 2: Cho$\Delta ABC$,

                                      CMR$\frac{a\sin A+b\sin B+c\sin C}{a\cos A+b\cos B+c\cos C}=cotA+cotB+cotC$

Edited by phuonglien99, 03-02-2014 - 18:26.