Consider the integral ∫x3x4+5 dx\int x^3 \sqrt{x^4+5} \,dx∫x3x4+5dx. Which substitution would be most appropriate?
u=x3u = x^3u=x3
u=x4+5u = x^4 + 5u=x4+5
u=x4+5u = \sqrt{x^4+5}u=x4+5
u=x4u = x^4u=x4