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March 28, 2024, 11:09:57 pm

Author Topic: Why can you take the positive only?  (Read 2089 times)  Share 

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Re: Why can you take the positive only?
« Reply #15 on: June 21, 2010, 09:43:31 pm »
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id solve the Q like this:

(x-1)^2 = (4-x)^2

|x-1| = |4-x|

|(x-1)/(4-x)|=0

solve for x this should be simple.

it's nothing to do with taking the positive only, split into cases, and solve it.

After |x-1| = |4-x|
can you just solve it:
x-1 = 4 - x or x-1 = -(4-x)
instead of dividing it? I don't really get why you divide it though...
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TrueTears

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Re: Why can you take the positive only?
« Reply #16 on: June 21, 2010, 09:48:02 pm »
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dividing is easier coz u can put it into a whole modulus and thus only need 2 cases

what you did is incorrect working:
Quote
can you just solve it:
x-1 = 4 - x or x-1 = -(4-x)

you missed 2 other cases.
« Last Edit: June 21, 2010, 09:53:48 pm by TrueTears »
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Re: Why can you take the positive only?
« Reply #17 on: June 21, 2010, 10:00:04 pm »
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which other cases?
-(x-1) = 4-x or -(x-1) = - (4-x)???
Wouldn't it be the same? :P
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TrueTears

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Re: Why can you take the positive only?
« Reply #18 on: June 21, 2010, 10:06:38 pm »
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yah, but you'd still have to state it, id follow the setting out for modulus in methods, u know, split each into hybrid and actually list out the possibilities, if you just go straight into it, sure it's fine from a pure mathematical point of view, but examiners wont know why u only picked 2, you should show them that the other 2 are redundant (like i said before) because they are same (or in some other cases, the domain does not satisfy the conditions). i know its trivial and stupid but its vce you should show all ur workings.
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TrueTears

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Re: Why can you take the positive only?
« Reply #19 on: June 21, 2010, 10:16:56 pm »
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well its like when ur eliminating solutions, its obvious sometimes one solution is redundant but still you would state why and then write the correct one, like the same here, 2 of them is redundant, so you should show like this: (i cant be bothered showing the whole thing but u get the point)

case 1

if x-1 =>0 and 4-x => 0

case 2

case 3

case 4

list the 4 different combinations, but clearly 2 of them is the same, so ud just solve the 2 other cases, and even when ur solving the other 2 cases, 1 of them again is redundant so again ud have to show it and then finally write the correct one. lame and trivial i know, because its vce maths.
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