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# Some shortcuts of math

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Manager
Joined: 27 Jul 2011
Posts: 181
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Kudos [?]: 176 [1] , given: 103

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17 Aug 2012, 18:45
1
KUDOS
This set is mixed one:(it is always good to approach formal way rather than to put some shortcut formulas....But why not give a try if it can save some time )

1. Area of a circle circumscribing equilateral triangle = $$\frac{pi}{3}a^2$$

2. Area of a circle inscribed in a equilateral triangle= $$\frac{pi}{12}a^2$$

3. Triangle formed by joining the mid points of a equilateral triangle will be half of perimeter and $$\frac{1}{4}area$$.

4.For a infinite GP series sum = $$\frac{a1}{(1-r)}$$ where a1 is the first term and r is multiplication factor

5. When Difference between compound interest and simple interest on a certain sum of money for 2 years at r% is
= $$\frac{(DIffX 100X100)}{(RateXRate)}$$

6. WHEN diff between CI and SI for 3 years at r% is given , then sum = $$\frac{Diff X100X100X100}{{(r^2)X(300+r)}}$$

7. If sum A becomes sum B in t1 years at compound rate of interest , then after t2 years the sum becomes

I am having a headache writing it properly - it is $${B^(t2/t1)} / { A^{(t2/t1)-1}}$$

8. Certain distance covered in xkm/hr and same distance y km/hr then avg speed = $$\frac{2xy}{x+y}$$

9. If two persons A and B start at the same time in opposite directions from two points and after passing each other they complete the journey
in a hrs and b hrs respectively . then A's speed : B's Speed = $$\sqrt{b} : \sqrt{a}$$

10.A man takes x hours to walk a certain place and ride back. However if he walks both ways he needs t hours more . Thus time taken
by him to ride both ways = $$(x-t)$$ hours.

I will post other tips if u like my post...(I mean If get few few kudos)...
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Senior Manager
Joined: 15 Sep 2009
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GMAT 1: 750 Q V
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Kudos [?]: 66 [0], given: 6

Re: Some shortcuts of math [#permalink]

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27 Aug 2012, 09:01
Good initiative but honestly you are better off understanding concepts than memorizing "non-standard" abstruse formulae. Many high-performing quant students tend to rely on understanding quant through basic first principles. Moreover, higher level (700+) questions on the GMAT are so based on simple math and flexibility in thinking that sometimes memorized formulas can be a handicap, as they tend to restrict your thinking and frustrate you when you realize that you can't simply plug and chug.

That said, I understand different test takers use different strategies, so to each his own.

Cheers,
Der alte Fritz.
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Re: Some shortcuts of math [#permalink]

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28 Aug 2012, 18:10
Personally, I agree with OldFritz, it is absolutely unnecessary to burden one's brain with abstruse formulas. For example, I am pretty certain that a very very small subsection of 99% scorers would know the expression for the Area of a circle circumscribing equilateral triangle. The GMAT doesn't really emphasize that. The only formula that I have ever used on the GMAT is the expression for the area of an equilateral triangle with side s, [sqrt(3)*s^2]/4. Other than that a problem solving mind set, along with the ability to think and approach a problem in multiple ways is probably more relevant for the GMAT.

Cheers,
Dabral
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Location: India
Concentration: Strategy, General Management
GMAT 1: 670 Q49 V33
WE: Asset Management (Manufacturing)
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Re: Some shortcuts of math [#permalink]

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10 Sep 2012, 14:14
Though memorizing a few formula are helpful in quickly answering, however memorizing complicated formulas is sometimes difficult, specially geometry related formulas.

For your first two formulas, I would prefer to derive this value from properties of triangle ( Heitht = $$\sqrt{3}* a/ 2$$) , instead of memorizing them.
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Re: Some shortcuts of math   [#permalink] 10 Sep 2012, 14:14
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