|x-1|-|x+2|<=0|x-1|<=|x+2|-(x+2)<=x-1<=x+2当-(x+2)<=x-1时-x-2<=x-12x>=-1x>=-1/2 1当x-1<=x+2时不管x为何值，不等式都有-1<=2成立 2∴结合1式2式得x>=-1/2
lx-1l≤lx+2l两边平方得x²-2x+1≤x²+4x+4=>x ≥-1/2
