根号(9-4根号2) =根号(8-4根号2+1) =根号[(2√2)²-2×2√2×1+1²] =根号[2(√2)-1]² =|2(√2)-1| =2(√2)-1
把大根号里的东西配个完全平方``````
根号下(9-4根号2)=√(8-2√8+1)=√(2√2-1)^2=2√2-1