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if A+B+C=180 then prove that cos^2a+cos^2b+2cosa*cosb*cosc=sin^2c

CHIKUN GARTIA , 9 Years ago
Grade 11
anser 1 Answers
Lab Bhattacharjee

Last Activity: 9 Years ago

\cos^2A-\sin^2C+2\cos A\cos B\cos C\\ =\cos(A+C)\cos(A-C)+2\cos A\cos B\cos C\\ =\cos(\pi-B)\cos(A-C)+2\cos A\cos B\cos C\\ =\cos B[-\cos(A-C)+2\cos A\cos C]\\ =\cos B[\cos(A+C)]\\ =\cos B[\cos(\pi-B)] \\ =?
 

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