resolver:
(a) dy/dx = 4x^3 * y^2y(1) = 1/3
(b) dy/dx = (x^2 + 1) e^yy(0) = 1/0
(c) dy/dx = sen (2x)yy(π/2) = 2
(d) dy/dx = e^3x √yy(0) = 4
