2x平方-3x+5
=2(x²-3x/2+5/2)
=2(x²-3x/2+9/16-9/16+5/2)
=2[(x-3/4)²+31/16]
=2(x-3/4+i√31/4)(x-3/4-i√31/4)=0
x1=(3-i√31)/4
x2=(3+i√31)/4