Question

1)cosx/2=-1
2)ctg(x+p/4)=-1

Answer 1

1)cosx/2=-1
x/2=pi+2piN
x=2pi+4piN
2)ctg(x+p/4)=-1
x+p/4=-p/4+pN
x=-p/2+pN

Answer 2

№1 
cos(x/2)= -1;
x/2=π+2πn;
x=2π+4πn;
№2
ctg(x+π/4)= -1;
x+π/4= 3π/4+πn;
x=π/2+πn

Answer 3

у вас ошибочка:3p/4-p/4=p/2