I understand the basic idea of getting the sq.rt off the bottom by multiplying top and bottom by the sq.rt but it doesn't always seemto work, is there more to it?

2/sq.rt8 -I get sq.rt8/4 , but answer says sq.rt2/2

a/ sq.rt40/2 -I get sq.rt4/8 -answer says sq.rt10a/10

And I am not sure how to tackle this one:

2/ 1+sq.rt6

Thanks.