方法1：
√5*[√5-√(1/5)]
=√5*√5-√5*√(1/5)
=5-1
=4
方法2：
√5*[√5-√(1/5)]
=√5*[√5-1/(√5)]
=√5*[√5-(√5)/5]
=√5*[(4√5)/5]
=4
注意：√(1/5)=（√1）/（√5)=1/(√5)=(√5)/5