GMATprep PS question

[mlane25269]

Hey guys!! Can someone explain? Thanks!

[Motherjane]

(sqrt 2 + sqrt 5) ^ 2 is of the form (a+b)^2 = a^2 + b^2 + 2 ab

or, 2 + 5 + 2( sqrt 2 * sqrt 5) = 7 + 2( sqrt 10)

sqrt 10 > sqrt 9 which is 3

so approximately the value will be : 7 + 2(3) = 13

[mlane25269]

Thanks for the explanation, MotherJane!

Is (a+b)^2 = a^2 + b^2 + 2 ab some kind of formula used for square root problems like this because I haven't seen it in any of the GMAT books I've read.