（x^4 + 2x^3 - (1/2)x^2 )÷(1/2)x^2
=x^4÷(1/2)x^2 + 2x^3 ÷ (1/2)x^2 - (1/2)x^2÷(1/2)x^2
=2x^2 + 4x - 1