sanju09 wrote:
Under these restrictions, no choice other than [spoiler]C[/spoiler] can take 90 a + 21, which happens at a = -2.
[spoiler]C[/spoiler]
I preferred to EXPLOIT the choices in hand, instead of going through the ways which are either long or beyond the GMAT specifications.
nice approach!
GMATSUCKER wrote:Any other approach to solve this problem ?
as said by sanju, the other methods to solve this problem will probably employ techniques that are beyond GMAT.
For e.g.
generic quad eq f(x) = ax^2 + bx + c (observe that there are 3 variables)
now based on the question, you can easily get value of C =1 and another eq. which gives a+b=2.
what about the third eq ?
you will have to differentiate the quadratic eq. with respect to x ( a concept beyond the scope), which will give you :
maximum value of this quad eq. is at x= (-b/2a). which on further solving will give a=-2 and b=4.
Hence the quad function is y= -2x^2 + 4x +1
at x=10, the value will be -159.
But, as a point to remember you can keep this in mind :
maximum/minimum value of a 2nd degree equation (quad function) in x represented by ax^2 + bx + c is at x= (-b)/2a
would like to see any simpler methods.
