Question Number 43875 by peter frank last updated on 16/Sep/18
$${if}\:\:\:\mathrm{tan}\:\left({x}+{y}\right)=\frac{\mathrm{3}}{\mathrm{4}}\:\:{and}\:\mathrm{tan}\left({x}−{y}\right)=\frac{\mathrm{8}}{\mathrm{15}}\: \\ $$$${show}\:{that}\:\:\mathrm{tan}\:\mathrm{2}{x}=\:\frac{\mathrm{77}}{\mathrm{36}} \\ $$
Answered by ajfour last updated on 16/Sep/18
$$\mathrm{tan}\:\left({x}+{y}+{x}−{y}\right)=\frac{\frac{\mathrm{3}}{\mathrm{4}}+\frac{\mathrm{8}}{\mathrm{15}}}{\mathrm{1}−\frac{\mathrm{2}}{\mathrm{5}}} \\ $$$$\Rightarrow\:\:\:\:\:\mathrm{tan}\:\mathrm{2}{x}\:=\:\frac{\mathrm{77}}{\mathrm{36}}\:. \\ $$