-4x^2+x+5=y
等价于-4（x^2-1/4x)+5=y
等价于-4[（x-1/8)^2-1/64]+5=y
等价于-4（x-1/8)^2+1/16+5=y
等价于-4（x-1/8)^2+81/16=y
完毕