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Geometry

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by BTGmoderatorRO » Fri Dec 29, 2017 7:22 am
A thin piece of wire 40 meters long is cut into two pieces. One piece is used to form a circle with radius r, and the other is used to form a square. No wire is left over. Which of the following represents the total area, in square meters, of the circular and the square regions in terms of r?

A. π∗r2π∗r2
B. π∗r2+10π∗r2+10
C. π∗r2+14∗π2∗r2π∗r2+14∗π2∗r2
D. π∗r2+(40−2π∗r)2π∗r2+(40−2π∗r)2
E. π∗r2+(10−12π∗r)2

OA is E

Do I need a solid shape formula to solve this? The help of an Expert is needed here, please.Thanks
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by Brent@GMATPrepNow » Fri Dec 29, 2017 7:31 am
A thin piece of wire 40 meters long is cut into two pieces. One piece is used to form a circle with radius r, and the other is used to form a square. No wire is left over. Which of the following represents the total area, in square meters, of the circular and the square regions in terms of r?

A. πr²
B. πr² +10
C. πr² + 1/4 π²r²
D. πr² + (40-2πr)²
E. πr² + (10 - (1/2)πr)²
One approach is to plug in a value for r and see what the output should be.

Let's say r = 0. That is, the radius of the circle = 0
This means, we use the entire 40-meter length of wire to create the square.
So, the 4 sides of this square will have length 10, which means the area = 100

So, when r = 0, the total area = 100

We'll now plug r = 0 into the 5 answer choices and see which one yields an output of 100

A) (pi)(0²) = 0 NOPE
B) (pi)(0²) + 10 = 10 NOPE
C) (pi)(0²) + 1/4([pi]² * 0²) = 0 NOPE
D) (pi)(0²) + (40 - 2[pi]0)² = 1600 NOPE
E) (pi)(0²) + (10 - 1/2[pi](0))² = 100 PERFECT!

Answer: E

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
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by Brent@GMATPrepNow » Fri Dec 29, 2017 7:35 am
A thin piece of wire 40 meters long is cut into two pieces. One piece is used to form a circle with radius r, and the other is used to form a square. No wire is left over. Which of the following represents the total area, in square meters, of the circular and the square regions in terms of r?

A. πr²
B. πr² +10
C. πr² + 1/4 π²r²
D. πr² + (40-2πr)²
E. πr² + (10 - (1/2)πr)²
Here's an algebraic approach:

Since r is the radius of the circle, the area of the circle will be πr²

If r is the radius of the circle, the length of wire used for this circle will equal its circumference which is 2Ï€r

So, the length of wire to be used for the square must equal 40 - 2Ï€r

In other words, the perimeter of the SQUARE will be 40 - 2Ï€r

Since squares have 4 equal sides, the length of each side of the square will be [40 - 2Ï€r]/4, which simplifies to be 10 - (Ï€r/2)

If each side of the square has length 10 - (πr/2), the area of the square will be [10 - (πr/2)]²

So, the total area will equal πr² + [10 - (πr/2)]², which is the same as E

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
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by Scott@TargetTestPrep » Mon Sep 02, 2019 6:34 pm
A thin piece of wire 40 meters long is cut into two pieces. One piece is used to form a circle with radius r, and the other is used to form a square. No wire is left over. Which of the following represents the total area, in square meters, of the circular and the square regions in terms of r?

A. πr²
B. πr² +10
C. πr² + 1/4 π²r²
D. πr² + (40-2πr)²
E. πr² + (10 - (1/2)πr)²
[/quote]

We are given that a thin piece of wire 40 meters long is cut into two pieces. One piece is used to form a circle with radius R, and the other is used to form a square.

Since the circumference of a circle with radius R is 2Ï€R, the length of wire used to form the circle is 2Ï€R. Thus, we have (40 - 2Ï€R) left over to form the square. In other words, the perimeter of the square is (40 - 2Ï€R). However, since we need to calculate the total area of the circular and the square regions, we need to determine the side of the square in terms of R. Since the perimeter of the square is (40 - 2Ï€R), the side of the square is:

side = (40 - 2Ï€R)/4

side = 10 - (1/2)Ï€R

Now we can determine the areas of the circle and the square.

Area of circle = πR2

Area of square = side^2 = (10 - (1/2)Ï€R)^2

Thus, the combined area of the circle and square is πR2 + (10 - (1/2)πR)2.

Answer: E

Scott Woodbury-Stewart
Founder and CEO
[email protected]

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