\[ sin^2x+cos^2x=1 \]
\[ cos^2x=1-sin^2x \]
\[ cosx=\pm \sqrt{1-sin^2x} \]
\[ sin^2x=1-cos^2x \]
\[ sinx=\pm \sqrt{1-cos^2x} \]
\[ tgx \cdot ctgx=1 \]
\[ sin2 \alpha=2sin \alpha \cdot cos \alpha \]
\[ cos2\alpha=cos^2\alpha – sin^2\alpha \]
\[ cos2\alpha=1 -2 sin^2\alpha \]
\[ cos2\alpha=2 cos^2\alpha – 1 \]
