Q (3) Prove that
We know that
and
We use these two in our problem
and
= +
= 1 + 2cosxcosy + 1 - 2sinxsiny
= 2 + 2(cosxcosy - sinxsiny)
= 2 + 2cos(x + y)
= 2(1 + cos(x + y) )
Now we can write
=
= R.H.S.