③4x²+12x+9=0（2x+3）²=0x=-3/2④x²+2x+1=3（x+1）²=3x+1=±√3x=±√3-1⑤3x²+4x-7=0（x-1）（3x+7）=0x=1或x=-7/3
2a分之负b加减根号b方减4ac
