Answer:
We have to find cos2 A in terms of m
and n. This means that the angle B is to be eliminate from the
given relations.
Now,
and
(1)
On substituting the values of
and
in
we get
(1)
(1)
(1)
Hence Proved
OR
We have,
and
and
?(i)
and
?(ii)(1)
Adding Eqs. (i) and (ii), we get
(1/2)
Putting
in
Eq. (ii), we get
?(iii)(1/2)
In
?(iv)(1/2)
Adding Eq. (iii) and (iv), we get
Putting the value of C in Eq. (iv), we get
(1/2)
Hence,
(1/2)
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