You start by looking intensely to the equation for a few minutes; you'll surely notice that something is misplaced:
[math](x+1)(x+3)(x+5)(x+7)=9[/math]
looks much better. Now note that [math]1+7=8=3+5[/math]. Symmetry is the key! If we set [math]y=x+4[/math]...
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