thangvietnam wrote:

how to pick the number ? picking number is time consuming.

any tip, trick here,

Hi,

When we look at \(f(x,y)=3x^2-2xy+y^2+4\) and then at the options, we find that plugging values is the best way to approach this question.

There are 3 things that should be keptp in mind while picking option values.

1.Pick integers first. They are easy to work on.

There are 3 values in the option list, which are integers.

2.Pick ‘0’ first. This will eliminate one variable completely for compuation.

3.What to choose first; x or y? One should always observe right hand side of the function. If number of terms of x is more than the number of terms of y, then plug in the option value in x first, and vice versa.We choose the values for x in the order of 0, 1, and -1 to plug in.

Now, we plug in the value of f(x, y) =8, & x=0 in the equation, and we get,

\(8=3.0^2-2.0.y+y^2+4\)

\(8=y^2+4\)

\(4=y^2\)

y= ±2This means for x=0, y is either 2 or -2. There is no such option available for values: 2 or-2 , hence these pair of values cannot be correct.

Now, we should try x=1. You may follow PraPon’s solution for x=1. He has done it correctly.

Hope it helps!

-Shalabh

_________________

Aiming to score 760+ on the GMAT? Attend our free webinar to learn how to define your GMAT strategy, create your study plan and master the core skills to excel on the GMAT. Click here to register for this webinar. The webinar will start at 7 AM PST on the 6th of June 2015.