Indices problem:Gmat prep-2

[gmat740]

[(9 +√80)^0.5 + ( 9 -√80)^0.5 ] ^2


a. 1
b.9 -4√5
c.18 - 4√5
d. 20
e.18

OA D

[kapsii]

using (a+b)^2 = a^2 + b^2 + 2.a.b.

((9 +√80)^.5)^2 + ((9 -√80)^.5)^2 + 2.[{(9 +√80)^.5} * {(9 -√80)^.5}]

or 18 + 2 [((9 + √80)*(9 - √80))^0.5]

(a+b)(a-b) = a^2 - b^2

= 18 + 2* [(81-80)^0.5]
= 18 + 2
= 20.

[gmat740]

Thanks a lot

I made such a foolish mistake