Help me with Riemann sum approximation? The expression ( ( 1/20 ) ( √1/20 √2/20 √3/20 ldots √20/20 ) ) is a Riemann sum approximation for:

a) (∫₀¹ √x , dx)
b) (∫₀¹ x √x , dx)
c) (∫₀¹ x² √x , dx)
d) (∫₀¹ x³ √x , dx)