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July 13th, 2018, 04:34 AM   #1
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Why can't I solve an equation this way?

I don't undertand why I can't solve the equation like I did in the circle. It produces a wrong answer.

In my understanding I don't violate a rule by multiplying 2, to get rid of the 2 in the denominator on the left side.
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 July 13th, 2018, 06:05 AM #2 Math Team   Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 13,608 Thanks: 954 You've posted "garbage"...please show EXACTLY the original equation. If 2 = (m/2) - (7/7), then: 2 = m/2 - 1 4 = m - 2 m = 6
 July 13th, 2018, 06:06 AM #3 Global Moderator   Joined: Dec 2006 Posts: 19,974 Thanks: 1850 You're misunderstanding the notation, Denis. When multiplying or dividing, each term must be used, so you should get 4 = m - 14 (which leads to 18 = m, which is correct).
 July 13th, 2018, 06:15 AM #4 Senior Member   Joined: Apr 2014 From: Glasgow Posts: 2,132 Thanks: 717 Math Focus: Physics, mathematical modelling, numerical and computational solutions $\displaystyle 2 = \frac{m}{2} - 7$ Multiply both sides by 2: $\displaystyle 2 \times 2 = 2 \times \left(\frac{m}{2} - 7\right)$ $\displaystyle 4 = 2 \left(\frac{m}{2} - 7\right)$ Expand brackets: $\displaystyle 4 = 2 \times \frac{m}{2} - 2 \times 7$ $\displaystyle 4 = \cancel{2} \times \frac{m}{\cancel{2}} - 2 \times 7$ $\displaystyle 4 = m - 14$
 July 13th, 2018, 06:16 AM #5 Math Team   Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 13,608 Thanks: 954 Huh? I'm looking at the one on right; I'm assuming it means : 2 = m/2 - 7/7 Then I'm assuming the OP is trying to solve that....
July 13th, 2018, 10:46 AM   #6
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Quote:
 Originally Posted by Denis Huh? I'm looking at the one on right; I'm assuming it means : 2 = m/2 - 7/7 Then I'm assuming the OP is trying to solve that....
Except that then the rest of what is done is non-sense! Also there is a "+" in front of the 7 on the right. I interpret the "/+7" as meaning that he has the equation 2= m/2- 7 and that he is adding 7 to both sides. That would give 9= m/2 as on the next line.

 July 13th, 2018, 10:55 AM #7 Math Team   Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 13,608 Thanks: 954 Frankly my dear, I don't give a damn
 July 13th, 2018, 12:39 PM #8 Math Team   Joined: Jan 2015 From: Alabama Posts: 3,261 Thanks: 894 Your a stern man!
July 13th, 2018, 01:41 PM   #9
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Quote:
 Originally Posted by skipjack You're misunderstanding the notation, Denis. When multiplying or dividing, each term must be used, so you should get 4 = m - 14 (which leads to 18 = m, which is correct).
I must admit that I cannot make heads or tails out of the notation either.

$2 = \dfrac{m}{2} - \dfrac{7}{.2} \implies 4 = m - \dfrac{14}{.2} \implies$

$m = 4 + \dfrac{14}{.2} = 4 + \dfrac{140}{2} = 74.$

Now if that /. is supposed to mean multiply 2 times both sides of the equation

$2 = \dfrac{m}{2} - 7 \not \implies 4 = m - 7 \implies m = 11.$

Is this standard notation somewhere and the OP simply does not know that
$2 \times 7 \ne 7?$

July 13th, 2018, 01:53 PM   #10
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Quote:
 Originally Posted by Country Boy Your a stern man!
I'm more a utern guy

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