x1＋x2=-b/a,x1·x2=c/a
﹙1﹚2x²-x-1=0
x1＋x2=1/2,x1·x2=-1/2
﹙2﹚x(2x-5)=4x-10→2x²-9x＋10=0
x1＋x2=9/2,x1·x2=5
﹙3﹚3x²-5x=0
x1＋x2=5/3,x1·x2=0
﹙4﹚2x²+5x+7=-3+11→2x²＋5x-1=0
x1＋x2=-5/2,x1·x2=-1/2