9+4√2=8+4√2+1=(2√2+1)²∴2√2+1=x+2√y∵x，y均为有理数∴x=1 y=2∴2x-y=0∴立方根（2x-y）=0
√（9+4√2）=√(8+(4√2)+1)=√((2√2)²+2*2(√2)+1)=√((2√2)+1)²=(2√2)+1√（9+4√2）=x+2√yx=1y=2³√(2x-y)=0
