以后限定方法请事先说
(1)2x^2-7x-5=0
x^2-7x/2=5/2
x^2-7x/2+(7/4)^2=5/2+(7/4)^2
(x-7/2)^2=89/16
x-7/2=±√89/4
x=(7±√89)/4
(2)4x^2+9x+3=0
x^2+9x/4=-3/4
x^2+9x/4+(9/8)^2=-3/4+(9/8)^2
(x+9/8)^2=33/64
x+9/8=±√33/8
x=(-9±√33)/8
(3)5x^2-2x-4=0
x^2-2x/5=4/5
x^2-2x/5+(1/5)^2=4/5+(1/5)^2
(x-1/5)^2=21/25
x-1/5=±√21/5
x=(-1±√21)/5