Given,

Now,

Now,

Now,

Now,

xy + yz + xz

{∵ cos (A + B) = cos A cos B – sin A sin B}

{sin (180° - A) = sin A ; sin (180° + A) = -sin A;

cos (180° - A) = -cos A & cos (180° + A) = -cos A }

= 0

= R.H.S.

Hence Proved