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Difficult numbers

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by vini1612 » Mon Sep 19, 2011 8:02 am
Hi,

I came across a question like this on one of my tests:

What is the value of the equation:
2x^2 - 2.4x - 1.7 where x = 0.7
a) -1.42
b) -2.40
c) -0.40
d) -1.70
e) -2.20

Now, although the question is simple, i was not able to solve it correctly due to time constraints. After substituting the value of x in the equation, the multiplication becomes very complex. Is there a simple way of solving this problem? i tried the method of approximation but that didnt work for me!
Any help will be appreciated since I am really struggling with time in the maths section :)

Thanks everyone!
Vini
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by sl750 » Mon Sep 19, 2011 8:12 am
You could replace x=0.7 with 7/10. That makes the calculation simpler

2*49/100 -2.4*7/10 -1.7

LCM is 100

(98-168-170)/100 = -2.4
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by gmatclubmember » Mon Sep 19, 2011 8:16 am
This would be slightly simpler:
2x(x-1.2)-1.7 = 2(.7)(-1/2)-1.7=-.7-1.7=-2.4
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by Abhishek009 » Mon Sep 19, 2011 9:36 am
vini1612 wrote:Hi,

I came across a question like this on one of my tests:

What is the value of the equation:
2x^2 - 2.4x - 1.7 where x = 0.7
a) -1.42
b) -2.40
c) -0.40
d) -1.70
e) -2.20

Now, although the question is simple, i was not able to solve it correctly due to time constraints. After substituting the value of x in the equation, the multiplication becomes very complex. Is there a simple way of solving this problem? i tried the method of approximation but that didnt work for me!
Any help will be appreciated since I am really struggling with time in the maths section :)

Thanks everyone!
Vini
Same approach as sl750 has shown :

2x^2 - 2.4x - 1.7


2(7/10)^2 - 2.4(7/10) - 1.7

0.98 - 1.68 - 1.7

0.98 - 3.38

- 2.40
Abhishek
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by vini1612 » Mon Sep 19, 2011 11:42 am
Thanks everyone! this helps..
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by LalaB » Tue Sep 20, 2011 4:27 am
for a lazy one like me hehe
if we round 0.7 , we get 1 .so lets assume x=1
then
2x^2 - 2.4x - 1.7

2*1^2 -2.4*1 -1.7 =2.1

then I just check out answer choices. which choice is near to 2.1, this one is ok
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by schreze » Tue Sep 20, 2011 5:38 am
i think the answer choices are not too close together, so you can work with estimates

x^2 =~0.5 (0.49)
2.4x = ~1.7

so 1-1.7-1.7 = -2.4
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