I think that the question made a typo and put dx istead of dt.
And du/dt is indeed 4t^3
Don't forget to change the limits when you make the substitution.
That's what I thought too as he hasn't taught us how to do this. He has taught us and stuff like that but if I do it that way, it doesn't give me a specific value. I look at the graph and it's apparent that the area should be zero, but yeah...must be a typo.
Originally Posted by Hardwork
I know what you are thinking, but the integral is with respect to x, not t.
This is just Calc 1, have you seen problem/s like this before?