Prove the following:
, 
, 
We know that: cos (x - y) = cosx cosy + sinx siny ...(i)
sin (x - y) = sinx cosy - cosx siny ...(ii)
...(iii)
....(iv)
Replacing x by 
and y by x in (i), we get
= - sinx [By using (iii) and (iv)]
Hence, 
Replacing x by 
and y by x in (ii), we get
= - cosx [By using (iii) and (iv)]
Hence, 
Hence, 
