Show that, `4 R r cos ""A/2cos ""B/2 cos ""C/2 =Delta`

If r and R are radii of the incircle and circum-circle of DeltaABC, then prove that : 8rR{cos ^(2) A//2 +cos ^(2) B//2 +cos ^(2) C//2} =2bc+2ca+2ab-a^(2) -b^(2) -c^(2).

cos 4x = cos 2x

With usual notatins, if in a Delta ABC,(b+c)/(11)=(c+a)/(12) =(a+b)/(13), then prove that (cos A)/(7) =(cos B)/(19) =(cos C)/(25).

In a Delta ABC, c cos ^(2)""(A)/(2) + a cos ^(2) ""(C ) /(2)=(3b)/(2), then a,b,c are in

If sinA=1/2 , show that 3 cos A - 4 cos^(3) A= 0

A line makes the angle alpha, beta , gamma and delta with the diagonals of a cube. The cos^2 alpha + cos^2 beta + cos^2 gamma + cos^2 delta =.............

In a triangle ABC, cos 3A + cos 3B + cos 3C = 1 , then then one angle must be exactly equal to

Solve b cos ^(2) ""(C)/(2) +c cos ^(2) ""(B)/(2) in terms of k, where k is permeter of the DeltaABC.

If cos alpha, cos beta, gamma are direciton cosines then cos 2 alpha + cos 2 beta + cos 2 gamma = ................

If A,B,C are the angles of a triangle then prove that cosA+cosB-cosC=-1+4cos(A/2)cos(B/2)sin(C/2)