F(x) =-2/3x^3+2x^2-x
f ( x ) = - 2 / 3 * x ^ 3 + 2 * x ^ 2 - x ;
f ' ( x ) = ( - 2 / 3 * x ^ 3 + 2 * x ^ 2 - x ) ' = - 2 / 3 * 3 * x ^ ( 3 - 1) + 2 * 2 * x ^ ( 2 - 1 ) - 1 = - 2 * x ^ 2 + 4 * x - 1 ;