Y=2x^3 is nonlinear, since it's a cube equation. y=x+5 is linear, because it's a line with y intercept (0,-5) and slope 1. y=\sqrt(x-5) is nonlinear, because it's a quadratic equation when reversing x and y. 5x+5y=25 is equal to x+y=5 ot y=-x+5, which is also linear. y=x^2/2 is a quadratic equation, so it's nonlinear.
