韦达定理
a+b=6
ab=4
[a^(1/2)+b^(1/2)]²
=a+b+2(ab)^(1/2)
=6+2×4^(1/2)
=10
a^(1/2)+b^(1/2)>0
所以a^(1/2)+b^(1/2)=√10