<s>[INST] $a^2-b^2=bc$ and $b^2-c^2=ac \Rightarrow a^2-c^2=ab$ Some weeks ago our math teacher asked the following question and gave us a week to solve it: 

If $a^2-b^2=bc$ and  $b^2-c^2=ac ,$ Prove $a^2-c^2=ab$, Where $a,b,c$ are non-zero real numbers.

This seemed really easy at the first, but when i tried to prove it i just failed every time. After a week, I only came up with this idea: Assume our case is true. $a^2-c^2=ab\Rightarrow a^2-b^2+b^2-c^2=ab\Rightarrow bc+ac=ab\Rightarrow\frac{1}{abc}(bc+ac)=\frac{1}{abc}(ab) \Rightarrow$ $\frac{1}{a}+\frac{1}{b}=\frac{1}{c}$. Now if we Prove this, our case will be proved as well.
  In my opinion this seemed like a really nice question so i wanted to share it with everyone.
 [/INST] To begin with, note that by adding the two given equations together, we can immediately conclude that $$a^2-c^2=(a+b)c.\tag{$\star$}$$
Now, multiply both sides of $(\star)$ by $a-b,$ giving us $$(a^2-c^2)(a-b)=(a^2-b^2)c\\a^3-a^2b-ac^2+bc^2=bc^2\\a^3-a^2b-ac^2=0\\a(a^2-ab-c^2)=0,$$ so since $a\ne 0,$ we can conclude that $a^2-ab-c^2=0,$ so that $a^2-c^2=ab,$ as desired.
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