y = (√x / 3)(3-x),dy/dx = (1-x)/(2√x),(dy/dx)² = (1-x)²/(4x)The arc length = ∫[1,3] √[1 + (dy/dx)²] dx= ∫[1,3] √[1 + (1-x)²/(4x)] dx= ∫[1,3] √[(4x+1-2x+x²)/(4x)] dx= ...