Cosine of a double angle NMT \[\cos2\alpha=\cos^2\alpha-\sin^2\alpha\] \[\cos2\alpha=2\cos^2\alpha-1\] \[\cos2\alpha=1-2\sin^2\alpha\] 10–11 grade
Formulas for 90°+α NMT \[\sin(90^\circ+\alpha)=\cos\alpha\] \[\cos(90^\circ+\alpha)=-\sin\alpha\] \[\tg(90^\circ+\alpha)=-\frac1{\tg\alpha}\] 10–11 grade
Formulas for 180°−α NMT \[\sin(180^\circ-\alpha)=\sin\alpha\] \[\cos(180^\circ-\alpha)=-\cos\alpha\] \[\tg(180^\circ-\alpha)=-\tg\alpha\] 10–11 grade