y = ∫(x→4) √(1 + t²) dt
dy/dx = 0 - √(1 + x²)
dy = - √(1 + x²) dx
y = ∫(1→x²) 1/(1 + t) dt
dy/dx = dx²/dx * 1/(1 + x²) - 0
dy/dx = 2x/(1 + x²)