sin\alpha=\frac{\sqrt2}{2} dla kąta 45 st
to
cos\alpha=\frac{\sqrt2}{2}
lub można obl z jedynki tryg
sin^2\alpha+cos^2\alph=1
cos\alpha=\sqrt{1-\frac{1}{2}}=\frac{\sqrt2}{2}
3cos^2\alpha-2sn^2\alpha=3(\frac{\sqrt2}{2})^2-2*(\frac{\sqrt2}{2})^2=
=3*\frac{2}{4}-2*\frac{2}{4}=\frac{3}{2}-1=\frac{1}{2}