How to do this? 3√90- √10

Question

 do not know how to do this either 

Answer 1

90 = 9 * 10 = 3*3*2*5

         

sqrt(90) = sqrt(3)*sqrt(3)*sqrt(2)*sqrt(5)

            = 3 * sqrt(10)  <---- sqrt(3)*sqrt(3) = 3 and sqrt(2)*sqrt(5) = sqrt(10)

 

So the result is

 

3*3*sqrt(10) - sqrt(10) =

9*sqrt(10) - sqrt(10) =

8*sqrt(10)