Question

Вычислите ctg2a,если tga=1,2​

Answer 1

$\mathrm{ctg}\,2a=1:\mathrm{tg}\,2a =1:\dfrac{2\,\mathrm{tg}\,a}{1-\mathrm{tg}\,^2a}=\dfrac{1-\mathrm{tg}^2a}{2\,\mathrm{tg}\,a}=\\\\=\dfrac{1-1.2^2}{2\cdot1.2}=\dfrac{1-1.44}{2.4}=\dfrac{-0.44}{2.4} =-\dfrac{0.44}{2.4} =-\dfrac{44}{240} =-\dfrac{11}{60}$