Question

The equation $a^7xy-a^6y-a^5x=a^4(b^4-1)$ is equivalent to the equation $(a^mx-a^n)(a^py-a^2)=a^4b^4$ for some integers $m$, $n$, and $p$. Find $mnp$.

Answer 1

Let simplify the equations $a^7xy-a^6y-a^5x=a^4(b^4-1)$ and $(a^mx-a^n)(a^py-a^2)=a^4b^4$:

1) $a^7xy-a^6y-a^5x+a^4=a^4b^4$

and

2) $a^{m+p}xy-a^{n+p}y-a^{m+2}x+a^{n+2}=a^4b^4$.

Equate the coefficients:

$xy: m+p=7 \\ y: n+p=6 \\ x: m+2=5 \\ 1: n+2=4$.
Then $n=2 \\ p=4 \\ m=3$ and mnp=24.