Prove :

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Then We have to prove Square both members: Square again both members: which is true.

cos A = tan A cos A - sin A / cos A = 0 cos^2 A - sin A = 0 (1 - sin^2 A) - sin A = 0 sin^2 A + sin A - 1 = 0 sin A = (-1 +/- (1 - 4(1)(11))^(1/2)/[2(1)] sin A = (-1 +/- sqrt 5)/2 A = 45 degress = pi/4 rad that means if A = 0 to A<pi/4, cos A > tan A

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