NCERT - If of cord = 0 then prove I. ac cos "- + cos2 OC cosi = INCERTI

ABC is a triangle and D is the middle point of BC. If AD is perpendicular to AC, then prove that cos A . cos C =(2(c^(2)-a^(2)))/(3ac)

If A+B+C=0 , then prove that cos ^(2)A+ cos ^(2) B + cos ^(2) C =1+2 cos A cos B cos C

Prove that: cos 100^0+cos 20^0=cos 40^0

In any triangle, ABC prove that: ac cos B-bc cos A=a^2-b^2

In sinA=sinB and cos A=cos B , then prove that sin(A-B)/(2)=0

In sinA=sinB and cos A=cos B , then prove that sin((A-B)/2)=0

If cosx + cosy + cosz = 0 and sinx + siny + sinz =0 , then show that cos (x-y) + cos(y - z) + cos(z - x) = - 3/2.

If cosx + cosy + cosz = 0 "and" sinx + siny + sinz =0, "then show that" cos (x-y) + cos(y - z) + cos(z - x) = - 3/2.

If cos x. cosy+sinx.siny=-1 then cosx+cosy is :