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I was watching this video: https://www.youtube.com/watch?v=aSId_Kgjzfg&index=55&list=PLg2tfDG3Ww4vrstKAZ0dajHx_hq85P0G

At 10:25 I saw the following fractions subtracted on the third line going upwards:

-1/8 -152/8 = 153/8

Why is the answer not in negative form?

securityauditor
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  • Beat's me. What you wrote it should be. I'll take a peek at the video and see if there is something in context. – fleablood Sep 05 '18 at 20:10
  • @fleablood Should the answer be positive or negative? – securityauditor Sep 05 '18 at 20:10
  • Ah, in the video he was "bringing them to the other side of the equation". So he was adding $\frac 18 + \frac {152}8$ to both sides of the equation. – fleablood Sep 05 '18 at 20:11
  • The answer is negative. However, in the linked video, they also put the fraction on the other side of an equals sign, which has the effect of changing the sign from negative to positive. – davidlowryduda Sep 05 '18 at 20:11
  • This is a good reminder that you should pay attention to context. "The answer" depends on what question you are asking and if you are ever looking for "the answer" you should first make sure you ask yourself "the answer to what?" – JMoravitz Sep 05 '18 at 20:47

2 Answers2

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If you have $X - a =0$ then you add $a$ to both sides of the equation to get.

$X -a + a = 0 + a$

$X = a$

So in the video he has

$2(x+ \frac 14)^2 - \frac 18 - \frac {152}8 = 0$.

then he did the following steps in his head without writing them on the board.

...$2(x+ \frac 14)^2 - \frac {153}8 = 0$....

...$2(x+ \frac 14)^2 - \frac {153}8+ \frac {153}8= 0+ \frac {153}8$...

And concluded

$2(x+\frac 14)^2 = \frac {153}8$.

fleablood
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Because $2(x + \frac{1}{4})^2 - \frac{1}{8} - \frac{152}{8}= 0$ if and only if $2(x + \frac{1}{4})^2 = \frac{1}{8} + \frac{152}{8}=\frac{153}{8}$. Note that the right hand side changes at this step which is why the term is now positive.

CyclotomicField
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