p=r

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uptowngirl92
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Topic: p=r
PostSun Nov 01, 2009 3:06 pm Reply with quote
1/p > r /r^2 +2
the question is asking:
r^2 +2 > pr ??

From stmt 1 we know:
r^2 +2 > r^2 ..so why s the answer not A
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rohan_vus
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PostSun Nov 01, 2009 4:04 pm Reply with quote
Hi

You are making classic mistake of assuming p > 0 when working with this inequality.


1/p > r /r^2 +2
the question is asking:
r^2 +2 > pr ?? ----> Only if p > 0 if p < 0 then r^2 + 2 < pr.. When you cross multiply for inequalities the inequality sign remains as it is when +ve and reverses when you cross multiply with -ve number
===

Henc IMO C
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mehravikas
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PostTue Nov 03, 2009 7:23 pm Reply with quote
IMO - C

Salotuin same as above
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geemat
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PostTue Nov 03, 2009 8:57 pm Reply with quote
C for me too...
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palvarez
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PostFri Nov 06, 2009 8:52 pm Reply with quote
uptowngirl92 wrote:
1/p > r /r^2 +2
This is how you shud attack these beasts.

Simplify (1/p) - (r/r^2+2)
(r^2+2-pr)/[p(r^2+2)]

A = p(r^2 + 2 - pr), is A > 0

1. p = r
2. r > 0


1. p = r, A = 2p. Whether A is positive or not depends upon whether p is +ve or not, which we dont know. Insufficient.

2. r > 0, we cant determine A's sign using just r.

Combining together A = 2p = 2r > 0. Sufficient.
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heshamelaziry
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PostMon Nov 09, 2009 4:23 pm Reply with quote
I solve this by picking numbers, although not the best way. I didn't simplify because I made a rule for myself after making many mistakes:

If there are two variables on different sides of inequality, the only time I can cross multiply or divide each side by one of the variables, if I know that both variables are positive or both are negative.

Please tell me if I am wrong, and if I need to adjust this rule.

fo example: if there is w^3 > w^2 I can't divide each side by w.
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palvarez
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PostMon Nov 09, 2009 4:31 pm Reply with quote
heshamelaziry wrote:
I solve this by picking numbers, although not the best way. I didn't simplify because I made a rule for myself after making many mistakes:

If there are two variables on different sides of inequality, the only time I can cross multiply or divide each side by one of the variables, if I know that both variables are positive or both are negative.

Please tell me if I am wrong, and if I need to adjust this rule.

fo example: if there is w^3 > w^2 I can't divide each side by w.
w^3 > w^2

Just make sure about the sign of the number you are dividing.

for instance, w^2 is +ve whetehr w is +ve or -ve.

Therfore, w > 1.

w^4 > w^3

Here, you can't divide by w^3. Just divide by w^2.

You are left with: w^2 > w
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