Question 28554
{{{cos^2(A) - sin^2(A) }}}


now, {{{cos^2(A) + sin^2(A) = 1 }}}
so that {{{cos^2(A) = 1 - sin^2(A) }}}


so, put that into the top equation, giving:
{{{1 - sin^2(A) - sin^2(A) }}}


which becomes {{{1 - 2sin^2(A) }}} which is the other side of the Question... so this proves it.


jon.