Question

вычислить cos165 градусов

Answer 1

cos165⁰=cos(180⁰-165⁰)=-cos15⁰;⇒cos²15⁰=(1+cos30⁰)/2;
cos15⁰=√(1+√3/2)/2=+₋√(2+√3)/4;
cos165⁰=-[√(2+√3)]/4;