Prove that:
We know that
       Â
Replacing  by x, we get
             cos(x + x) = cosx. cosx - sinx. sinx
∴              cos 2x =                               ...(A)
Also, we know that Â
∴                                               ...(i)
                                                ...(ii)
Putting (i) in (A), we get
                Â
                                               ...(B)
Putting (ii) in (A), we get
                Â
                                               ...(C)
Now, Â Â Â Â Â Â Â Â Â Â Â Â Â
∴                                          ....(D)
From (A), (B), (C) and (D), we have
      Â
            .
     Â