ctg7π/3=ctg(2π+π/3)=ctgπ/3=√3/3

cos17π/4=cos(4π+π/4)=cosπ/4=√2/2.

sin(-25π/6)=-sin(4π+π/6)= -sinπ/6=- 1/2.

см.формулы приведения и периодичность триг. функций