Let I be `I=int(7x^2 sin (pi x) dx`
Integrate it by part
`u(x)=7x^2 u'(x)=14x`
`v'(x)=sin pi x v(x)=-1/(pi)cos pi x`
`I=-7/pi x^2cos (pi x) -int 14x*(-1/(pi)cos pi x) dx`
`I=-7/pi x^2cos (pi x) +14/pi int x*cos pi x) dx`
Integrate it by part again
`u=x u'(x)=1`
`v'(x)=cos(pi x), v(x)=1/(pi) sin pi x`
`I=-7/pi x^2cos (pi x) +14/(pi) (x/pi) sin pi x-int(1/pi) sin pi x dx)`
`I=-7/pi x^2cos (pi x) +14/(pi)^2 x sin pi x- 14/(pi)^2 int sin pi x dx`
Answer:
`I=-7/pi x^2cos (pi x) +14/(pi)^2 x sin pi x+14/(pi)^3cos pi x +C `
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