设t=√2x+1(t≥0),则x=1/2(t2-1).于是y=1/2(t2-1)-
设t=√2x+1(t≥0),则
x=1/2(t2-1).
于是y=1/2(t2-1)-3+t=1/2(t+1)2-4≥1/2-4=-7/2.最后这步不应该等于-4吗