`a)`
`sqrt((84^2-37^2)/47)=sqrt(((84-37)(84+37))/47)`
`=sqrt((47.121)/47)=sqrt121=11`
`b)`
Đặt `A=sqrt(3+sqrt5)-sqrt(3-sqrt5)-sqrt2`
`sqrt2A=sqrt2(sqrt(3+sqrt5)-sqrt(3-sqrt5)-sqrt2)`
`sqrt2A=sqrt(6+2sqrt5)-sqrt(6-2sqrt5)-2`
`sqrt2A=sqrt(5+2sqrt5+1)-sqrt(5-2sqrt5+1)-2`
`sqrt2A=sqrt((sqrt5+1)^2)-sqrt((sqrt5-1)^2)-2`
`sqrt2A=sqrt5+1-(sqrt5-1)-2`
`sqrt2A=sqrt5+1-sqrt5+1-2=0`
`A=0/(sqrt2)=0`