A_1,\quad A'_1,\quad \alpha_1,\quad \beta_2

a^2+b^2=c^2

\frac{a}{2}+\frac{b}{3}= \frac{3\cdot a+2\cdot b}{6}

T_∆ =\frac{ab\, \sin\, \gamma}{2}= \frac{a^2\, \sin\,\beta\, \sin\,\gamma}{2\, \sin\,\alpha}

tg\, \frac{\alpha}{2}= \pm\sqrt{\frac{1-\cos\,\alpha}{1+\cos\,\alpha}}

\int c\cdot f(x)\, dx= c\cdot \int f(x)\, dx

\int_a^b f(x)\, dx= \int_a^c f(x)\, dx+ \int_c^b f(x)\, dx