> f4:= x -> (tan(x) - 1) * ( 2 * sin(x) + 1);

> plot(f4(x), x = 0.. 2 * Pi, color = black);

> solve(f4(x) = 0, x );

> solve(tan(x) - 1 = 0, x);

> with(plots):

> a:= plot(2 * sin(x) + 1 , x = -Pi..2*Pi, color = black):

> b:= plot([-Pi/6,t,t=-1..1], color = red):

> c:= plot([(11/6)*Pi,t, t = -1..1], color = red):

> d:= plot([(7/6)*Pi,t ,t = -1..1], color = red):

> display({a,b,c,d});