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The above figure shows a sector of a circle. What is the area of the s

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Math Revolution GMAT Instructor
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The above figure shows a sector of a circle. What is the area of the s  [#permalink]

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11 Oct 2018, 00:35
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[Math Revolution GMAT math practice question]

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The above figure shows a sector of a circle. What is the area of the sector?

$$1) x = 120^o.$$
$$2) AB=6√3.$$

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"Only $149 for 3 month Online Course" "Free Resources-30 day online access & Diagnostic Test" "Unlimited Access to over 120 free video lessons - try it yourself" Director Joined: 18 Jul 2018 Posts: 679 Location: India Concentration: Finance, Marketing WE: Engineering (Energy and Utilities) Re: The above figure shows a sector of a circle. What is the area of the s [#permalink] Show Tags 11 Oct 2018, 00:41 1 From statement 1: Area of a sector is given by pi*$$r^2$$*$$\frac{120}{360}$$ only angle is given. no info about the radius. Hence insufficient. From statement 2: Length of the Arc is given. No info about the angle. Insufficient. Combining both: The area can be found. C is the answer. _________________ Press +1 Kudo If my post helps! VP Joined: 31 Oct 2013 Posts: 1162 Concentration: Accounting, Finance GPA: 3.68 WE: Analyst (Accounting) Re: The above figure shows a sector of a circle. What is the area of the s [#permalink] Show Tags 11 Oct 2018, 01:14 1 MathRevolution wrote: [Math Revolution GMAT math practice question] Attachment: 11.11.png The above figure shows a sector of a circle. What is the area of the sector? $$1) x = 120^o.$$ $$2) AB=6√3.$$ Statement 1: $$x= 120^0$$. Clearly NOT sufficient . This information can be useful if we know the total area or part of the area of the circle. Statement 2: AB =6√3 . Using this information , all we can do is to find out the total area. AB is basically diameter here. As we don't know the ration of % of this sector , This pieces of information alone is useless. NOT sufficient. Combining both statements : we have total area from statement 2 and the % from statement 1. Sufficient. The best answer is C. GMATH Teacher Status: GMATH founder Joined: 12 Oct 2010 Posts: 772 Re: The above figure shows a sector of a circle. What is the area of the s [#permalink] Show Tags 11 Oct 2018, 06:32 MathRevolution wrote: [Math Revolution GMAT math practice question] Attachment: 11.11.png The above figure shows a sector of a circle. What is the area of the sector? $$1) x = 120^o$$ $$2) AB=6√3$$ (We assume - as part of the definition of a sector a circle - that the "origin" of the angle x shown in the figure is the center of the circle.) The variable R will denote the radius of the circle. All angles are measured in degrees. $$? = \frac{{x\,}}{{360\,}}\left( {\pi {R^{\,2}}} \right)$$ $$x = 120\,\,\,\left( {{\text{both}}\,\,{\text{figures}}} \right)$$ $$AB = 6\sqrt 3 \,\,\,\left( {{\text{both}}\,\,{\text{figures}}} \right)$$ $$x = 90\,\,\,\mathop {\,\,\, \Rightarrow \,\,\,\,}\limits^{L\,,\,L\,,\,L\sqrt 2 } \,\,\,R\sqrt 2 = 6\sqrt 3 \,\,\,\,\,\,\mathop \Rightarrow \limits^{ \cdot \,\,\frac{{\sqrt 2 }}{2}\,} \,\,\,\,\,R = 3\sqrt 6 \,\,\,\,\,\,\,\, \Rightarrow \,\,\,\,\,\,\,\,\,?\,\,\, = \,\,\,\frac{1}{4}\,\left( {\pi \, \cdot 9 \cdot 6} \right)$$ $$x = 120\,\,\,\mathop \Rightarrow \limits_{\left( * \right)}^{30\,,\,60\,,\,90\,\,} \,\,\,\,?\,\,\, = \,\,\,\frac{1}{3}\,\left( {\pi \, \cdot 36} \right)\,\,\, = 12\pi \,\,\, \ne \,\,\,\,\frac{1}{4}\,\left( {\pi \, \cdot 9 \cdot 6} \right)\,\,\,\,\,\,$$ $$\left( * \right)\,\,30\,,\,60\,,\,90\,\,\,\, \Rightarrow \,\,\,\,\left\{ \begin{gathered} \,L\sqrt 3 \,\,\, = \,\,\frac{{6\sqrt 3 }}{2} \hfill \\ \,2L = R \hfill \\ \end{gathered} \right.\,\,\,\,\,\,\,\,\, \Rightarrow \,\,\,\,\,\,R = 6$$ $$\left( {1 + 2} \right)\,\,\,?\,\, = \,\,12\pi \,\,\,\left( {{\text{shown}}\,\,{\text{above}}} \right)$$ This solution follows the notations and rationale taught in the GMATH method. Regards, Fabio. _________________ Fabio Skilnik :: GMATH method creator (Math for the GMAT) Our high-level "quant" preparation starts here: https://gmath.net Math Revolution GMAT Instructor Joined: 16 Aug 2015 Posts: 6985 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: The above figure shows a sector of a circle. What is the area of the s [#permalink] Show Tags 14 Oct 2018, 17:56 => Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution. Attachment: 10.15.png [ 9.94 KiB | Viewed 444 times ] The area of a sector is $$(\frac{1}{2}) πr^2*(\frac{x}{360})$$, where $$r$$ is the radius of the sector. Since we have two variables, $$r$$ and $$x$$, C is most likely to be the answer and we need to check both conditions together first. Conditions 1) and 2): Quadrilateral $$OACB$$ is a kite, so its diagonals bisect each other at right angles, and bisect the angles at the vertices. Since $$AB = 6√3, AD = \frac{AB}{2} = 3√3$$. Since $$x =120^o$$, angle $$AOD$$ has measure $$60^o$$, the triangle $$ODA$$ is a right triangle and $$OD:OA:DA = 1:2:√3$$. This yields $$OA:DA = r: 3√3 = 2: √3,$$which implies that $$r = 6.$$ Thus, the area of the sector is$$(\frac{1}{2}) π6^2*(\frac{120}{360}) = (\frac{1}{2}) (\frac{1}{3})36π = 6π.$$ Both conditions together are sufficient. Therefore, C is the answer. Answer: C Normally, in problems which require 2 equations, such as those in which the original conditions include 2 variables, or 3 variables and 1 equation, or 4 variables and 2 equations, each of conditions 1) and 2) provide an additional equation. In these problems, the two key possibilities are that C is the answer (with probability 70%), and E is the answer (with probability 25%). Thus, there is only a 5% chance that A, B or D is the answer. This occurs in common mistake types 3 and 4. Since C (both conditions together are sufficient) is the most likely answer, we save time by first checking whether conditions 1) and 2) are sufficient, when taken together. Obviously, there may be cases in which the answer is A, B, D or E, but if conditions 1) and 2) are NOT sufficient when taken together, the answer must be E. _________________ MathRevolution: Finish GMAT Quant Section with 10 minutes to spare The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy. "Only$149 for 3 month Online Course"
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Re: The above figure shows a sector of a circle. What is the area of the s  [#permalink]

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15 Oct 2018, 11:06
MathRevolution wrote:
The area of a sector is $$(\frac{1}{2}) πr^2*(\frac{x}{360})$$, where $$r$$ is the radius of the sector.

$$πr^2$$ is the area of the circle, and $$\frac{x}{360}$$ ($$x$$ in degrees) is the fraction of the area we are interested in. (Think about a slice of pizza.)

In short: "1/2" should not be present in the formula quoted.

Regards,
Fabio.
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Re: The above figure shows a sector of a circle. What is the area of the s  [#permalink]

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16 Oct 2018, 09:02
Dear fskilnik, could you please explain a few things ? Why do you take a sector with 90 degrees ? And then what is this notation : ⇒(∗)30,60,90 and where does the 1/3 * (π⋅36) come from ? I don't really understand your explanations.

Thanks a lot for your time ~
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The above figure shows a sector of a circle. What is the area of the s  [#permalink]

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16 Oct 2018, 09:50
EricD28 wrote:
Dear fskilnik, could you please explain a few things ? Why do you take a sector with 90 degrees ? And then what is this notation : ⇒(∗)30,60,90 and where does the 1/3 * (π⋅36) come from ? I don't really understand your explanations.

Thanks a lot for your time ~

Hi, EricD28 !

Thank you for your interest in my solution.

> Question 01: when considering statement (2) ALONE, we must obey the AB given length, but we are allowed to choose different values for x.
I chose first 90 degrees, then 120 degrees. These are particular cases especially easy to deal with the corresponding calculations.
The calculations were necessary to *guarantee* we can find two different numerical values for the question asked. This is what we call a BIFURCATION.

> Question 02: this notation means "using the 30-60-90 triangle shortcut, we may conclude that..."
(This shortcut is carefully explained in our method and, I believe, in almost every GMAT course).

> Question 03: 1/3 comes from the 120/360 degrees ratio, while pi*(6^2) is the area of the circle with radius 6.

Regards,
Fabio.
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Fabio Skilnik :: GMATH method creator (Math for the GMAT)
Our high-level "quant" preparation starts here: https://gmath.net

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Re: The above figure shows a sector of a circle. What is the area of the s  [#permalink]

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17 Oct 2018, 08:03
Thank you very much Fabio, i solved using sin 60 but it is the same as the 30 60 90 (i didn't know this theorem, thx for the tip)

Have a good day
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Re: The above figure shows a sector of a circle. What is the area of the s  [#permalink]

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17 Oct 2018, 11:12
EricD28 wrote:
Thank you very much Fabio, i solved using sin 60 but it is the same as the 30 60 90 (i didn't know this theorem, thx for the tip)

Have a good day

Hi, EricD28.

YES, you may use basic trigonometry to justify the shortcut´s validity.
Another possibility: think about the height of an equilateral triangle, obtained when you "double" your 30-60-90 (along the hypotenuse).

Regards,
Fabio.
_________________

Fabio Skilnik :: GMATH method creator (Math for the GMAT)
Our high-level "quant" preparation starts here: https://gmath.net

Re: The above figure shows a sector of a circle. What is the area of the s   [#permalink] 17 Oct 2018, 11:12
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