急問附加數問題

you have the formula ...... simply sub
b=2
a=0
f(x) = l sin(x)

I guess your problem is how to integrate [sin(x)]^2 ...
Here you have to lower the degree by using cos(2x) = 1 - 2 [sin(x)]^2
∫[sin(x)]^2 dx
=(1/2) ∫1 - cos(2x) dx
=(1/2) [ x - (1/2) sin(2x)] + C

Thus, the root mean square value
= (1 / 2) (1/2) [ 2 - (1/2)sin(4)] (l^2)
= (1/8) [ 4 - sin(4)] (l^2)