hi, here there are different approaches I think
One option is to resolve the sq
is 20 the answer?
(a +b)^2 = a^2 + 2ab +b^2
So you get 9 + 9 + 2(a+b)(a-b)
and the last term is equal to a^2 - b^2 = 81-80
18 + 2 = 20

maybe I can write down with formulas if it is not clear

moliver wrote:hi, here there are different approaches I think
One option is to resolve the sq
is 20 the answer?
(a +b)^2 = a^2 + 2ab +b^2
So you get 9 + 9 + 2(a+b)(a-b)
and the last term is equal to a^2 - b^2 = 81-80
18 + 2 = 20

maybe I can write down with formulas if it is not clear

OA is 20 !

I am sorry I didn't quite get your explanation. What is the formula for solving such equations?

From your explanation:

I am aware abt this one:(a +b)^2 = a^2 + 2ab +b^2

so in our case ... what is a and what is b here ?


Please let me know. Thanks !!

Sorry gmatguy81, I didn't want to write down all.
a= (9 +80^(1/2))^(1/2)
b= (9 -80^(1/2))^(1/2)

we eliminate the root for the terms a^2 and b^2 and we have the other term that is: 2ab =
(9 +80^(1/2))^(1/2) * (9 -80^(1/2))^(1/2)
we can multiple what is inside of the root and make a big root
and what is inside of the root has the form of: (a+b)(a-b) with is equal to a^2 - b^2 so
{(9 +80^(1/2)) * (9 -80^(1/2))}^(1/2)
= (9*9-80)^(1/2)

so putting all together, we have 9+9+2*1= 20

I hope this make more sense.
Please let me know if you need further explanation, I would be glad to help you.