(a) ∫x3e-x
= – x3e-x + 3∫x2e-x dx ….(i)
∫x2 e-x dx
= – x2e-x + ∫2xe-x dx
= – x2e-x + 2∫xe-x dx ……(ii)
∫x e-x dx
= – xe-x + ∫e-x dx
= – xe-x – e-x + C …..(iii)
समी. (i), (ii) व (iii) को हल करने पर
∫x3e-x dx = – x3e-x + 3[- x2e-x + 2 ∫xe-x dx]
= -x3e-x + 3(-x2e-x + 2(-xe-x – e-x + C1))
= -x3e-x – 3x2e-x + 6(-xe-x – e-x + C1)
= -x3e-x – 3x2e-x – 6xe-x – 6e-x + 6C1
= -x3e-x – 3x2e-x – 6xe-x – 6e-x + C
= -e [x3 + 3x2 + 6x + 6] + C
(b) ∫x3 sinx
= -x3 cos x + 3x2 sin x – 6(-x cos x + ∫cos x dx)
= -x3 cosx + 3x2 sin x + 6x cos x – 6 sin x + C