Power Play


Given that, $p = 3^w$, $q = 3^x$, $r = 3^y$, $s = 3^z$, where $w, x, y, z$ are positive integers. Find the minimum value of $(w + x + y + z)$ such that $p^2 + q^3 + r^5 = s^7$.

$\left[a^m \times a^n = a^{m+n}, \frac{a^m}{a^n} = a^{m-n}\right]$


Source: BdMO 2016 National Junior P6


Proof Based Problems  


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Solution

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Given,

$p=3^w,q=3^x,r=3^y,s=3^z$ and $p^2+q^3+r^5=s^7$


So, $3^{2w}+3^{3x}+3^{5y}=3^{7z}$


Let, $a=2w,b=3x,c=5y,d=7z$

So $3^a+3^b+3^c=3^d$


Without loss of generality, we assume $a \leq b \leq c $

Then $3^a(1+3^{b-a}+3^{c-a})=3^d$


Now right hand side is a power of $3$.

So no other prime than $3$ divides left hand side

So $a=b=c$

So $3 \times 3^a=3^d$

$3^{a+1}=3^d$

So $a+1=d$

So $2w=3x=5y=7z-1$


To find the minimum value of $w+x+y+z$,

We have to find the minimum number $n$ such n is divisible by $2,3,5$ and $n+1$ is divisible by $7$

We find such $n$ is $90$.

So $w=45,x=30,y=18,z=13$

So, minimum $w+x+y+z=45+30+18+13=106$

This is a proof based problem added for learning purposes and does not accept submissions.

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