So I'm doing length of an arc in my calculus 1 class. After plugging everything in the arc length formula. Now I have this complicated function to integrate. Square root of (16x^8+8x^4+1)/16x^4. I took the denominator out of my square root and got 4x^2. Now I take u=4x^2. Du/2x =dx. Now my question is are we allowed to take the u substitution of a value in the denominator and not have to write it as 1/what we are sub? . The problem is even so no solution. But it seemed to follow correctly if my intuition was correct.