f(x) is continuous in [a,b]

here h=(b-a)/n

S=cos(a)+ cos(a+h)+ cos(a+2h)+ cos(a+3h)+…………………..+ cos(a+(n-1)h)

Putting h=(b-a)/n

As we know

Which is trigonometry formula of sin(b)-sin(a)

Final answer is sin(b)-sin(a)