根号2a加b=4
根号2a=4-b
2a=(4-b)^2
根号下a^2-2a+1-根号下b^2-8b+16
=根号下(a-1)^2-根号下(b-4)^2
=根号下(a-1)^2-根号下(4-b)^2
=a-1-根号2a