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# Patricia builds two triangles, each with 30 feet of wood. The first

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Math Expert
Joined: 02 Sep 2009
Posts: 47920
Patricia builds two triangles, each with 30 feet of wood. The first  [#permalink]

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23 Nov 2016, 06:55
2
5
00:00

Difficulty:

45% (medium)

Question Stats:

63% (02:04) correct 38% (01:30) wrong based on 104 sessions

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Patricia builds two triangles, each with 30 feet of wood. The first triangle ABC is built to maximize the length of the base side. The second triangle DEF is built to maximize the area of the triangle. What is the ratio of the length of the base of triangle ABC to the length of the base of triangle DEF? The lengths of all line segments are integers.

A. 1:1
B. 6:5
C. 5:4
D. 7:5
E. 2:1

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Re: Patricia builds two triangles, each with 30 feet of wood. The first  [#permalink]

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23 Nov 2016, 09:14
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Bunuel wrote:
Patricia builds two triangles, each with 30 feet of wood. The first triangle ABC is built to maximize the length of the base side. The second triangle DEF is built to maximize the area of the triangle. What is the ratio of the length of the base of triangle ABC to the length of the base of triangle DEF? The lengths of all line segments are integers.

A. 1:1
B. 6:5
C. 5:4
D. 7:5
E. 2:1

The lengths of 3 sides of triangle ABC is a, b, c where a, b, c are integers and c is the base side.
We have $$c<a+b \implies 2c < a+b+c=30 \implies c<15$$. Triangle ABC is built to maximize $$c$$, so $$c=14$$. The length of base side is 14.

For any triangle that has the same perimeter, equilateral triangle has maximum area. So triangle DEF has 3 sides $$d=e=f=10$$. The length of base side is 10.

Hence, the ratio is 14 : 10 = 7 : 5

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Posts: 20
Re: Patricia builds two triangles, each with 30 feet of wood. The first  [#permalink]

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23 Nov 2016, 11:53
How can you say the equilateral triangle has the maximum area? Pls clarify

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Re: Patricia builds two triangles, each with 30 feet of wood. The first  [#permalink]

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23 Nov 2016, 18:52
ashishahujasham wrote:
How can you say the equilateral triangle has the maximum area? Pls clarify

Sent from my A1601 using GMAT Club Forum mobile app

I use Heron's formula:

$$S_{ABC}=\sqrt{p(p-a)(p-b)(p-c)}$$ where $$p=\frac{a+b+c}{2}=const$$

From inequation: $$xyz \leq ( \frac{x+y+z}{3})^3$$ for $$x, y, z>0$$. Sign $$"="$$ occurs $$\iff x=y=z$$

We have $$S_{ABC} \leq \sqrt{p \times \left ( \frac{p-a+p-b+p-c}{3} \right) ^3}= \sqrt{p \times (\frac{p}{3})^3}=\frac{p^2}{3\sqrt{3}}$$

$$maxS_{ABC}=\frac{p^2}{3\sqrt{3}} \iff a=b=c=\frac{2p}{3}$$
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Re: Patricia builds two triangles, each with 30 feet of wood. The first  [#permalink]

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14 Dec 2017, 11:43
1
broall wrote:
Bunuel wrote:
Patricia builds two triangles, each with 30 feet of wood. The first triangle ABC is built to maximize the length of the base side. The second triangle DEF is built to maximize the area of the triangle. What is the ratio of the length of the base of triangle ABC to the length of the base of triangle DEF? The lengths of all line segments are integers.

A. 1:1
B. 6:5
C. 5:4
D. 7:5
E. 2:1

The lengths of 3 sides of triangle ABC is a, b, c where a, b, c are integers and c is the base side.
We have $$c<a+b \implies 2c < a+b+c=30 \implies c<15$$. Triangle ABC is built to maximize $$c$$, so $$c=14$$. The length of base side is 14.

For any triangle that has the same perimeter, equilateral triangle has maximum area. So triangle DEF has 3 sides $$d=e=f=10$$. The length of base side is 10.

Hence, the ratio is 14 : 10 = 7 : 5

Where do you get the 2c from? I can't figure out why
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Re: Patricia builds two triangles, each with 30 feet of wood. The first  [#permalink]

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14 Dec 2017, 18:14
rnz wrote:
broall wrote:
Bunuel wrote:
Patricia builds two triangles, each with 30 feet of wood. The first triangle ABC is built to maximize the length of the base side. The second triangle DEF is built to maximize the area of the triangle. What is the ratio of the length of the base of triangle ABC to the length of the base of triangle DEF? The lengths of all line segments are integers.

A. 1:1
B. 6:5
C. 5:4
D. 7:5
E. 2:1

The lengths of 3 sides of triangle ABC is a, b, c where a, b, c are integers and c is the base side.
We have $$c<a+b \implies 2c < a+b+c=30 \implies c<15$$. Triangle ABC is built to maximize $$c$$, so $$c=14$$. The length of base side is 14.

For any triangle that has the same perimeter, equilateral triangle has maximum area. So triangle DEF has 3 sides $$d=e=f=10$$. The length of base side is 10.

Hence, the ratio is 14 : 10 = 7 : 5

Where do you get the 2c from? I can't figure out why

c < a + b => c + c < a + b+ c => 2c < a + b + c
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Posts: 6521
Re: Patricia builds two triangles, each with 30 feet of wood. The first  [#permalink]

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14 Dec 2017, 20:20
Bunuel wrote:
Patricia builds two triangles, each with 30 feet of wood. The first triangle ABC is built to maximize the length of the base side. The second triangle DEF is built to maximize the area of the triangle. What is the ratio of the length of the base of triangle ABC to the length of the base of triangle DEF? The lengths of all line segments are integers.

A. 1:1
B. 6:5
C. 5:4
D. 7:5
E. 2:1

QUICK tip:-
MAX base will be 14 since 15 or more will make it a LINE and NOT a triangle..
so in given answers ONLY D is with a 7 in it..
without any further calculations you can press D as the answer..

otherwise if you are doing untimed, .. The area of equilateral triangle is MAX in given perimeter and similarly a SQUARE in quadrilateral..
REASONING - closer the number, MORE the product

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1) Absolute modulus : http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
2)Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html
3) effects of arithmetic operations : https://gmatclub.com/forum/effects-of-arithmetic-operations-on-fractions-269413.html

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Re: Patricia builds two triangles, each with 30 feet of wood. The first &nbs [#permalink] 14 Dec 2017, 20:20
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