x平方-x-4=x平方-x＋1/4-17/4=(x-1/2)平方-17/4=(x-1/2+√17/2)(x-1/2-√17/2)
x^2-x-4=x^2-x+1/4-17/4=(x-1/2)^2-17/4=(x-1/2+√17/2)(x-1/2-√17/2)
