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Did I solve the following quadratic equation correctly.

$$W(W+2)-7=2W(W-3)$$

I got.

$$W^2-8W+7$$ Then for my solution I got.

$$(W-1)(W-7)$$

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Your second and third expressions should have an "$=0$" in there, but yes. –  Cameron Buie Jun 19 '12 at 17:35
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Strictly, you want $(W-1)(W-7)=0$ [same for second line too] so that $W=1$ or $W=7$. The easiest way to check you haven't made a mistake is by substituting these values into the original equation and checking that they work. –  Mark Bennet Jun 19 '12 at 17:37
    
Omission of $=0$ is a frequent fault. –  Michael Hardy Jun 19 '12 at 17:39
    
@MichaelHardy: The OP just meant to check if the simplification is correct. I don't see any frequent faults. –  Gigili Jun 19 '12 at 17:40
    
@Gigili : I didn't mean frequent in this posting; I meant frequent out there in the world. Zillions of students make this same mistake. I think they think what they're doing is pushing symbols around according to prescribed rules, as in long division, rather than at each step making a statement that should be true. –  Michael Hardy Jun 19 '12 at 17:50
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1 Answer 1

$$W(W+2)-7=2W(W-3)$$

$$\Downarrow$$

$$W^2+2W-7=2W^2-6W$$

$$\Downarrow$$

$$W^2-8W+7=0$$

$$\Downarrow$$

$$(W-1)(W-7)=0$$

You're right, well done. The solutions are $W=7$ and $W=1$.

EDIT: It should be written as I showed above, you've omitted "$=0$" part of the equation, intentionally or unintentionally.

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Note that the arrows between the various displayed equations should be bidirectional. –  André Nicolas Jun 19 '12 at 18:01
    
@AndréNicolas: I don't think they should, since I'm showing one direction only. –  Gigili Jun 19 '12 at 18:03
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However, in principle you have only shown then that the original equation can have no roots other than (possibly) $1$ and/or $7$, but it doesn't show these are roots. Reversibility is key. (Or alternately check at the end that $1$ and $7$ actually satisfy the original.) The forward arrows only say that if $W$ is a root, then there are only two candidates. –  André Nicolas Jun 19 '12 at 18:08
    
@AndréNicolas: It isn't a "forward arrow", (what's that?) It's used for conclusion, I've learned it here on SE with the same usage. –  Gigili Jun 19 '12 at 18:12
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It is the implication symbol, and it is widely misused by students. –  André Nicolas Jun 19 '12 at 18:16
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