ISEE Upper Level Quantitative : Equilateral Triangles

Study concepts, example questions & explanations for ISEE Upper Level Quantitative

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Example Questions

Example Question #21 : Geometry

 is an equilateral triangle. Points  are the midpoints of , respectively.  is constructed.

Which is the greater quantity? 

(a) The perimeter of 

(b) Twice the perimeter of 

Possible Answers:

(a) is greater.

It is impossible to tell from the information given.

(a) and (b) are equal.

(b) is greater.

Correct answer:

(a) and (b) are equal.

Explanation:

If segments are constructed in which the endpoints form the midpoints of the sides of a triangle, then each of the sides of the smaller triangle is half as long as the side of the larger triangle that it does not touch. Therefore:

The perimeter of  is:

,

which is twice the perimeter of .

Note that the fact that the triangle is equilateral is irrelevant.

Example Question #21 : Geometry

Column A                 Column B

The perimeter           The perimeter

of a square with        of an equilateral

sides of 4 cm.           triangle with a side

                                        of 9 cm.

Possible Answers:

The quantity in Column B is greater.

There is not enough info to determine a relationship between the columns.

The quantities in both columns are equal.

The quantity in Column A is greater.

Correct answer:

The quantity in Column B is greater.

Explanation:

Perimeter involves adding up all of the sides of the shape. Therefore, the square's perimeter is or 16. An equialteral shape means that all of the sides are equal. Therefore, the perimeter of the triangle is or 27. Therefore, Column B is greater.

Example Question #1 : How To Find The Area Of An Equilateral Triangle

 is an equilateral triangle. Points  are the midpoints of , respectively.  is constructed.

Which is the greater quantity? 

(a) The area of 

(b) Twice the area of 

Possible Answers:

(b) is greater.

It is impossible to tell from the information given.

(a) is greater.

(a) and (b) are equal.

Correct answer:

(a) is greater.

Explanation:

If segments are constructed in which the endpoints form the midpoints of the sides of a triangle, then four triangles, congruent to each other and similar to the larger triangle, are formed. Therefore, one of these triangles - specifically,  - would have one-fourth the area of . This means  has more than twice the area of .

Note that the fact that the triangle is equilateral is irrelevant.

Example Question #1 : How To Find The Length Of The Side Of An Equilateral Triangle

Which of the following could be the three sidelengths of an equilateral triangle?

Possible Answers:

Correct answer:

Explanation:

By definition, an equilateral triangle has three sides of equal length. We can identify the equilateral triangle by converting the given sidelengths to the same units and comparing them.

We can eliminate the following by showing that at least two sidelengths differ.

 

2 yards =  feet.

Two sides have lengths 6 feet and 7 feet, so we can eliminate this choice.

 

4 feet =  inches

Two sides have lengths 48 inches and 50 inches, so we can eliminate this choice.

 

5 feet =  inches

Two sides have lengths 48 inches and 60 inches, so we can eliminate this choice.

 

 yards =  feet

Two sides have lengths 4 feet and 5 feet, so we can eliminate this choice.

 

 yards =  feet =  inches 

All three sides have the same length, making this the triangle equilateral. This choice is correct.

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