mgmat odd-ball question

[niraj_a]

this question is easy, but i made a simple mistake - used simultaneous equations at the end to cancel off. why did that not work?

If #p# = ap3+ bp – 1 where a and b are constants, and #-7# = 3, what is the value of #7#?

5
0
-2
-3
-5

OA -5

[stop@800]

#p# = ap3+ bp – 1

#-7# = 3

hence
a(-7)3+ b(-7) – 1 = 3

-a(7)3 - b(7) – 1 = 3

a(7)3 + b(7) + 1 = -3

a(7)3 + b(7) -1 + 2 = -3

a(7)3 + b(7) -1 = -5

#7# = -5

Hope this helps