سیگما

`Q=int sin(x)sin(2x)sin(3x) dx`
`Q=1/2 int (2sin(2x)sin(x))sin(3x) dx`
`Q=1/2 int (cos(2x-x)-cos(2x+x))sin(3x) dx`
`Q=1/2 int (cos(x)-cos(3x))sin(3x) dx`
`Q=1/4 int 3sin(3x)cos(x)-2sin(3x)cos(3x) dx`
`Q=1/4 int (sin(3x+x)+sin(3x-x))-sin(6x) dx`
`Q=1/4 int sin(4x)+sin(2x)-sin(6x) dx`
`Q=1/4 (-cos(4x)/4-cos(2x)/2+cos(6x)/6)`