If `A+B+C=180^@`,prove that `cos^2A+cos^2B+cos^2C=1-2cos Acos B cos C`.

Prove that 1-8sin^2Acos^2A=cos4A

Prove that 1+cos 2x=2cos^2x .

If A+B+C=pi . Then prove that cosA=-cos(B+C)

Prove that cos20^@cos 40^@=1/2(1/2+cos 20^@)

Prove that cos 20^@cos 80^@=1/2(1/2-cos 80^@)

If A+ B + C = pi, then cos^(2) A + cos ^(2) B + cos ^(2) C is equal to : A) 1 - cos A cos B cos C B) 1 - 2 cos A cos B cos C C) 2 cos A cos B cos C D) 1 + cos A cos B cos C

Prove that in triangle ABC , cos^(2)A+cos^(2)B-cos^(2)C=1-2sinAsinBcosC .

Prove that cos^2theta=(1+cos2theta)/2

If cos(A+B)sin(C+D)=cos(A-B)sin(C-D) , prove that cot A cot B cot C= cot D.