ISEE Upper Level Quantitative : How to find the area of a triangle

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

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Example Questions

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

Triangle B has a height that is twice that of Triangle A and a base that is one-half that of Triangle A. Which is the greater quantity?

(a) The area of Triangle A

(b) The area of Triangle B

Possible Answers:

It is impossible to tell from the information given.

(b) is greater.

(a) is greater.

(a) and (b) are equal.

Correct answer:

(a) and (b) are equal.

Explanation:

Let \(\displaystyle b\) and \(\displaystyle h\) be the base and height of Triangle A. Then the base and height of Triangle B are \(\displaystyle \frac{1}{2} b\) and \(\displaystyle 2h\), respectively.

(a) The area of Triangle A is \(\displaystyle A = \frac{1}{2} bh\).

(b) The area of Triangle B is \(\displaystyle A = \frac{1}{2}\cdot \frac{1}{2} b \cdot 2h = \frac{1}{2} bh\).

Therefore, (a) and (b) are equal.

Example Question #2 : How To Find The Area Of A Triangle

Two triangles on the coordinate plane have a vertex at the origin and a vertex at \(\displaystyle (c,d)\), where \(\displaystyle c > d>0\).

Triangle A has its third vertex at \(\displaystyle (10,0)\).

Triangle B has its third vertex at \(\displaystyle (0,10)\).

Which is the greater quantity?

(a) The area of Triangle A

(b) The area of Triangle B

Possible Answers:

It is impossible to tell from the information given

(b) is greater

(a) and (b) are equal

(a) is greater

Correct answer:

(b) is greater

Explanation:

(a) Triangle A has as its base the horizontal segment connecting \(\displaystyle (0,0)\) and \(\displaystyle (10,0)\), the length of which is 10. Its (vertical) altitude is the segment from \(\displaystyle (c,d)\) to this horizontal segment, which is part of the \(\displaystyle x\)-axis; its height is therefore the \(\displaystyle y\)-coordinate of this point, or \(\displaystyle d\)

The area of Triangle A is therefore \(\displaystyle A = \frac{1}{2} \cdot 10 \cdot d = 5d\)

(b) Triangle B has as its base the vertical segment connecting \(\displaystyle (0,0)\) and \(\displaystyle (0,10)\), the length of which is 10. Its (horizontal) altitude is the segment from \(\displaystyle (c,d)\) to this vertical segment, which is part of the \(\displaystyle y\)-axis; its height is therefore the \(\displaystyle x\)-coordinate of this point, or \(\displaystyle c\)

The area of Triangle B is therefore \(\displaystyle A = \frac{1}{2} \cdot 10 \cdot c = 5c\)

 

\(\displaystyle c > d\), so \(\displaystyle 5c > 5d\). (b), the area of Triangle B, is greater.

Example Question #2 : How To Find The Area Of A Triangle

A triangle has sides 30, 40, and 80. Give its area.

Possible Answers:

\(\displaystyle 75 \sqrt{105}\)

\(\displaystyle 600\)

None of the other responses is correct

\(\displaystyle 1,200\)

\(\displaystyle 1,600\)

Correct answer:

None of the other responses is correct

Explanation:

By the Triangle Inequality Theorem, the sum of the lengths of the two shorter sides of a triangle must exceed the length of its longest side. However, 

\(\displaystyle 30 + 40 = 70 < 80\);

Therefore, this triangle cannot exist, and the correct answer is "none of the other responses is correct".

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

Pentagon 2

The above depicts Square \(\displaystyle ABCD\)\(\displaystyle X, Y\), and \(\displaystyle Z\) are the midpoints of \(\displaystyle \overline{AD}\)\(\displaystyle \overline{BC}\), and \(\displaystyle \overline{AB}\), respectively. Which is the greater quantity?

(a) The area of \(\displaystyle \bigtriangleup BZY\)

(b) The area of \(\displaystyle \bigtriangleup ZXD\)

Possible Answers:

It is impossible to determine which is greater from the information given

(b) is the greater quantity

(a) and (b) are equal

(a) is the greater quantity

Correct answer:

(a) and (b) are equal

Explanation:

For the sake of simplicity, assume that the square has sidelength 2; this reasoning is independent of the actual sidelength.

Since \(\displaystyle X\)\(\displaystyle Y\), and \(\displaystyle Z\) are the midpoints of their respective sides, \(\displaystyle AZ = BZ= XD = BY = 1\), as shown in this diagram.

Pentagon 3

The area of \(\displaystyle \bigtriangleup BZY\), it being a right triangle, is half the product of the lengths of its legs: 

\(\displaystyle \frac{1}{2} \cdot BZ \cdot BY = \frac{1}{2} \cdot 1 \cdot 1 = \frac{1}{2}\)

The area of \(\displaystyle \bigtriangleup ZXD\) is half the product of the length of a base and the height. Using \(\displaystyle \overline{XD}\) as the base, and \(\displaystyle \overline{AZ}\) as an altitude:

\(\displaystyle \frac{1}{2} \cdot XD \cdot AZ= \frac{1}{2} \cdot 1 \cdot 1 = \frac{1}{2}\)

The two triangles have the same area.

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