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What is the value of 2x^2 - 2.4x - 1.7 for x = 0.7?

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by kvcpk » Thu Jun 17, 2010 10:42 am
Answer is D
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by kvcpk » Thu Jun 17, 2010 10:53 am
ironsferri wrote: What would be the strategy here?
Thank you
when x is in between 0 an 1, x^2 will be less than x.
so 2x^2 < 2.4x
so 2x^2 - 2.4x will be negative.
neg value -1.7 = value much less than 1.7. so A, B are out.

next, 2x^2 - 2.4x = 2x(x-1.2) = 2x(0.7-1.2) = -x = -0.7

so answer is -0.7 - 1.7 = -2.4
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by Stuart@KaplanGMAT » Thu Jun 17, 2010 11:17 am
ironsferri wrote:What is the value of 2x^2 - 2.4x - 1.7 for x = 0.7?

(A) -0.72
(B) -1.42
(C) -1.98
(D) -2.40
(E) -2.89


What would be the strategy here?

Thank you
Brute force is probably quickest - just substitute in for x. If you're comfortable with the math, it's a 15-30 second question.

2*(.7^2) - 2.4(.7) - 1.7 = 2*.49 - 1.68 - 1.7 = .98 - 3.38 = -2.4... choose (D).
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by kvcpk » Thu Jun 17, 2010 11:25 am
From my above post.. Iwould say.. best way to tackle is

2x^2 - 2.4x = 2x(x-1.2) = 2x(0.7-1.2) = -x = -0.7

so answer is -0.7 - 1.7 = -2.4
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by ironsferri » Thu Jun 17, 2010 11:30 am
Thanks guys for both your input - I think they are both very valuable !

Just a quick follow up question....may sound lame but why multiplying all for 10 (to make them all integers) doesn't work?

Thank you

C
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by kvcpk » Thu Jun 17, 2010 11:41 am
ironsferri wrote:Thanks guys for both your input - I think they are both very valuable !

Just a quick follow up question....may sound lame but why multiplying all for 10 (to make them all integers) doesn't work?

Thank you

C
You can multiply the equation by 10 and then divide the result by 10.
But you cannot multiply x by 10. because x^2, x in the eqtn will be different when x is multiplied by 10.
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by ironsferri » Fri Jun 18, 2010 9:45 am
That's why - you would assume you could unless you pay attention to that ^2.....
Thanks kvcpk!

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