cos2a=1-2sin^2a for all values of a.
\(cos2a= cos^{2} a- sin^{2} a \\ but \ cos^{2} a + sin^{2} a=1 \\ i.e. \ cos^{2} a=1- sin^{2} a\)Therefore,\(cos2a=1- sin^{2} a- sin^{2} a \\ =1-2 sin^{2} a\)
true (just did this on apex)
