原式=1-(a-b)/(a+2b)÷(a+b)(a-b)/(a+2b)²
=1-(a-b)/(a+2b)×(a+2b)²/(a+b)(a-b)
=1-(a+2b)/(a+b)
=(a+b-a-2b)/(a+b)
=-b/(a+b)
=-1/(√2+1)
=-√2+1