Bunuel
If \(x^2 \lt 81\) and \(y^2 \lt 25\), what is the largest prime number that can be equal to \(x-2y\)?
A. 3
B. 7
C. 11
D. 13
E. 17
\(x^2 \lt 81\) and \(y^2 \lt 25\) should help you understand that x is less than 9 and y is less than 5. But also note that x can be between -9 and 9 but not inclusive. Same is the case with y. -5<y<5.
\(x-2y\)= Prime Number
One thing is obvious we have to choose a negative value of y.
13 is easy to achieve. 7-(2*-3)=13.
But why we should lower x even though we can treat x as 8. Can we achieve 17? Remember that neither is supposed to be an integer.
8-(2*-4.5)
8+9
17
We can achieve 17. So answer is 17.
Note: Even if you are unable to figure out the value you should understand that it is achievable and should go ahead with option E.