Question

cos^2(5π+a)+ctga*sin2a , если a = 2π/3

Answer 1

$cos^{2}(5pi+a)+ctga*sin2a=cos^{2}a+frac{cosa*2sina*cosa}{sina}=$

 

$=cos^{2}a+2cos^{2}a=3cos^{2}a$

 

$3cos^{2}(frac{2pi}{3})=3*(-frac{1}{2})^{2}=3*frac{1}{4}=frac{3}{4}$