|x+1|+|x-1|
=|x+1|+|1-x|≥|x+1+1-x|=2
当(x+1)(1-x)≥0取等号,这里正好是取等号
所以(x+1)(1-x)≥0
(x+1)(x-1)≤0
-1≤x≤1