We know that

If θ is the acute angle between and , then

……(i)

(i) and

Here and

So, from (i), we have

……(ii)

⇒

⇒

And

Now,

⇒

⇒ By (ii), we have

⇒

⇒

(ii) and

Here, and

So, from (i), we have

…(iii)

⇒

⇒

And

Now,

⇒

⇒ By (iii), we have

⇒

⇒