a)
b)
c)
I'll do the second one.
Note that the identity
stands for all possible real numbers .
And by squaring up both sides of equation (2), we have
Therefore it's possible to cancel the term \sin^2x:
Therefore
That being said,
or
You're too late. The OP reposted b) and c) in this thread: http://www.mathhelpforum.com/math-he...ns-153161.html . No fault on your part -- she shouldn't have done that.