SSAT Upper Level Math : Properties of Triangles

Study concepts, example questions & explanations for SSAT Upper Level Math

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Example Questions

Example Question #131 : Properties Of Triangles

The lengths of a triangle with a perimeter of \(\displaystyle 121\) are \(\displaystyle 2x, 4x,\text{ and }5x\). Find the length of the longest side.

Possible Answers:

\(\displaystyle 22\)

\(\displaystyle 55\)

\(\displaystyle 44\)

\(\displaystyle 66\)

Correct answer:

\(\displaystyle 55\)

Explanation:

Add up all the sides to find the perimeter.

\(\displaystyle 2x+4x+5x=121\)

\(\displaystyle 11x=121\)

\(\displaystyle x=11\).

Plugging this value into the sides we get:

\(\displaystyle 2x, 4x, 5x \rightarrow 2(11), 4(11), 5(11)\)

The side lengths of the triangle are \(\displaystyle 22, 44, \text{ and 55}\).

The length of the longest side is \(\displaystyle 55\).

Example Question #1 : How To Find If Two Acute / Obtuse Triangles Are Similar

\(\displaystyle \bigtriangleup ABC \sim \bigtriangleup DEF\)\(\displaystyle AB = 24, BC = 30, AC = 36\)\(\displaystyle \bigtriangleup DEF\) has perimeter 400.

Which of the following is equal to \(\displaystyle DF \div DE\)?

Possible Answers:

\(\displaystyle 1.25\)

\(\displaystyle 1.2\)

\(\displaystyle 1.5\)

\(\displaystyle 0.8\overline{3}\)

\(\displaystyle 0.\overline{6}\)

Correct answer:

\(\displaystyle 1.5\)

Explanation:

The perimeter of \(\displaystyle \bigtriangleup DEF\) is actually irrelevant to this problem. Corresponding sides of similar triangles are in proportion, so use this to calculate \(\displaystyle DF \div DE\), or \(\displaystyle \frac{DF}{DE}\):

\(\displaystyle \frac{DF}{DE} = \frac{AC}{AB} = \frac{36}{24} = 1.5\)

 

 

Example Question #2 : How To Find If Two Acute / Obtuse Triangles Are Similar

\(\displaystyle \bigtriangleup ABC \sim \bigtriangleup DEF\)\(\displaystyle AB = 24, BC = 20, AC = 36\)\(\displaystyle \bigtriangleup DEF\) has perimeter 300.

Evaluate \(\displaystyle DE + EF\).

Possible Answers:

\(\displaystyle 225\)

Insufficient information is given to answer the problem.

\(\displaystyle 200\)

\(\displaystyle 165\)

\(\displaystyle 210\)

Correct answer:

\(\displaystyle 165\)

Explanation:

The ratio of the perimeters of two similar triangles is equal to the ratio of the lengths of a pair of corresponding sides. Therefore, 

\(\displaystyle \frac{DE}{AB}= \frac{P_{\bigtriangleup DEF}}{P_{\bigtriangleup ABC }}\) and \(\displaystyle \frac{EF}{BC}= \frac{P_{\bigtriangleup DEF}}{P_{\bigtriangleup ABC }}\), or

\(\displaystyle \frac{DE}{AB}=\frac{EF}{BC} = \frac{P_{\bigtriangleup DEF}}{P_{\bigtriangleup ABC }}\)

By one of the properties of proportions, it follows that

\(\displaystyle \frac{DE+EF}{AB+BC} = \frac{P_{\bigtriangleup DEF}}{P_{\bigtriangleup ABC }}\)

The perimeter of \(\displaystyle \bigtriangleup ABC\) is

\(\displaystyle AB+BC+ AC = 24+ 20+ 36 = 80\), so

\(\displaystyle \frac{DE+EF}{24+20} = \frac{300}{80}\)

\(\displaystyle \frac{DE+EF}{44} = \frac{300}{80}\)

\(\displaystyle \frac{DE+EF}{44} \cdot 44 = \frac{300}{80} \cdot 44\)

\(\displaystyle DE+EF=165\)

Example Question #1 : How To Find If Two Acute / Obtuse Triangles Are Similar

\(\displaystyle \bigtriangleup ABC \sim \bigtriangleup DEF\)\(\displaystyle AB= 12\)\(\displaystyle DE = 40\)\(\displaystyle \bigtriangleup DEF\) has perimeter 90.

Give the perimeter of \(\displaystyle \bigtriangleup ABC\).

Possible Answers:

\(\displaystyle 53\frac{1}{3}\)

\(\displaystyle 36\)

\(\displaystyle 360\)

\(\displaystyle 27\)

\(\displaystyle 300\)

Correct answer:

\(\displaystyle 27\)

Explanation:

The ratio of the perimeters of two similar triangles is the same as the ratio of the lengths of a pair of corresponding sides. Therefore, 

\(\displaystyle \frac{P_{\bigtriangleup ABC }}{P_{\bigtriangleup DEF}} = \frac{AB}{DE}\)

\(\displaystyle \frac{P_{\bigtriangleup ABC }}{90} = \frac{12}{40}\)

\(\displaystyle \frac{P_{\bigtriangleup ABC }}{90}\cdot 90 = \frac{12}{40} \cdot 90\)

\(\displaystyle P_{\bigtriangleup ABC} = 27\)

Example Question #2 : How To Find If Two Acute / Obtuse Triangles Are Similar

\(\displaystyle \bigtriangleup ABC \sim \bigtriangleup DEF\).

\(\displaystyle m\angle A + m\angle D = 94^{\circ }\)

\(\displaystyle m\angle B + m\angle F = 94^{\circ }\)

Evaluate \(\displaystyle m \angle C + m \angle E\).

Possible Answers:

\(\displaystyle 86^{\circ }\)

\(\displaystyle 188^{\circ }\)

These triangles cannot exist.

\(\displaystyle 172^{\circ }\)

\(\displaystyle 94^{\circ }\)

Correct answer:

\(\displaystyle 172^{\circ }\)

Explanation:

The similarity of the triangles is actually extraneous information here. The sum of the measures of a triangle is \(\displaystyle 180 ^{\circ }\), so:

\(\displaystyle m \angle A + m \angle B + m \angle C = 180^{\circ }\)

\(\displaystyle m \angle D + m \angle E + m \angle F = 180^{\circ }\)

\(\displaystyle m \angle A + m \angle B + m \angle C +m \angle D + m \angle E + m \angle F = 180^{\circ }+ 180^{\circ }\)

\(\displaystyle m \angle C + m \angle E + (m \angle A +m \angle D) + (m \angle B + m \angle F )= 360^{\circ }\)

\(\displaystyle m \angle C + m \angle E +94 ^{\circ } +94 ^{\circ }= 360^{\circ }\)

\(\displaystyle m \angle C + m \angle E +188 ^{\circ }= 360^{\circ }\)

\(\displaystyle m \angle C + m \angle E +188 ^{\circ }- 188^{\circ } = 360^{\circ } - 188^{\circ }\)

\(\displaystyle m \angle C + m \angle E = 172^{\circ }\)

Example Question #131 : Properties Of Triangles

Given: \(\displaystyle \bigtriangleup ABC\) and \(\displaystyle \bigtriangleup DEF\)\(\displaystyle \angle A \cong \angle D\) and \(\displaystyle \angle C \cong \angle F\).

Which of the following statements would not be enough, along with what is given, to prove that \(\displaystyle \bigtriangleup ABC \sim \bigtriangleup DEF\)?

Possible Answers:

\(\displaystyle \frac{AB}{DE} = \frac{BC}{EF}\)

\(\displaystyle \frac{AB}{DE} = \frac{AC}{DF}\)

The given information is enough to prove the triangles similar.

\(\displaystyle \frac{AC}{DF}= \frac{BC}{EF}\)

\(\displaystyle \angle B \cong \angle E\)

Correct answer:

The given information is enough to prove the triangles similar.

Explanation:

Two pairs of corresponding angles are stated to be congruent in the main body of the problem; it follows from the Angle-Angle Similarity Postulate that the triangles are similar. No further information is needed.

Example Question #132 : Properties Of Triangles

\(\displaystyle \bigtriangleup ABC \sim \bigtriangleup DEF\)\(\displaystyle \frac{AB}{DE} = \frac{25}{36}\). Which of the following is the ratio of the area of \(\displaystyle \bigtriangleup DEF\) to that of \(\displaystyle \bigtriangleup ABC\) ?

Possible Answers:

\(\displaystyle \frac{625}{1,296}\)

\(\displaystyle \frac{6}{5}\)

\(\displaystyle \frac{1,296}{625}\)

\(\displaystyle \frac{36}{25}\)

\(\displaystyle \frac{5}{6}\)

Correct answer:

\(\displaystyle \frac{1,296}{625}\)

Explanation:

 The similarity ratio of \(\displaystyle \bigtriangleup ABC\) to \(\displaystyle \bigtriangleup DEF\) is equal to the ratio of two corresponding sidelengths, which is given as \(\displaystyle \frac{25}{36}\); the similarity ratio of \(\displaystyle \bigtriangleup DEF\) to \(\displaystyle \bigtriangleup ABC\) is the reciprocal of this, or \(\displaystyle \frac{36}{25}\).

The ratio of the area of a figure to that of one to which it is similar is the square of the similarity ratio, so the ratio of the area of \(\displaystyle \bigtriangleup DEF\) to that of \(\displaystyle \bigtriangleup ABC\) is 

\(\displaystyle \left (\frac{36}{25} \right ) ^{2}= \frac{1,296}{625}\)

Example Question #5 : How To Find If Two Acute / Obtuse Triangles Are Similar

\(\displaystyle \bigtriangleup ABC \sim \bigtriangleup DEF\)\(\displaystyle m \angle D = 44 ^{\circ }, m \angle E = 26^{\circ }\) 

Which of the following is true about \(\displaystyle \bigtriangleup ABC\)?

Possible Answers:

\(\displaystyle \bigtriangleup ABC\) is scalene and acute.

None of the other responses is correct.

\(\displaystyle \bigtriangleup ABC\) is isosceles and acute.

\(\displaystyle \bigtriangleup ABC\) is scalene and obtuse.

\(\displaystyle \bigtriangleup ABC\) is isosceles and obtuse.

Correct answer:

\(\displaystyle \bigtriangleup ABC\) is scalene and obtuse.

Explanation:

Corresponding angles of similar triangles are congruent, so the measures of the angles of \(\displaystyle \bigtriangleup ABC\) are equal to those of \(\displaystyle \bigtriangleup DEF\).

Two of the angles of \(\displaystyle \bigtriangleup DEF\) have measures \(\displaystyle 44 ^{\circ }\) and \(\displaystyle 26^{\circ }\); its third angle measures 

\(\displaystyle 180^{\circ } - (44^{\circ }+ 26^{\circ } ) = 180^{\circ } - 70 ^{\circ } = 110^{\circ }\).

One of the angles having measure greater than \(\displaystyle 90^{\circ }\) makes \(\displaystyle \bigtriangleup DEF\) - and, consequently, \(\displaystyle \bigtriangleup ABC\) - an obtuse triangle. Also, the three angles have different measures, so the sides do as well, making \(\displaystyle \bigtriangleup ABC\) scalene.

Example Question #1 : How To Find If Two Acute / Obtuse Triangles Are Similar

\(\displaystyle \bigtriangleup ABC \sim \bigtriangleup DEF\)\(\displaystyle m \angle A = 66^{\circ }\)\(\displaystyle m \angle F = 63^{\circ }\).

Which of the following correctly gives the relationship of the angles of \(\displaystyle \bigtriangleup ABC\) ?

Possible Answers:

\(\displaystyle m \angle C =m \angle B < m \angle A\)

\(\displaystyle m \angle C < m \angle B < m \angle A\)

\(\displaystyle m \angle C < m \angle B = m \angle A\)

\(\displaystyle m \angle B < m \angle C < m \angle A\)

\(\displaystyle m \angle B = m \angle C < m \angle A\)

Correct answer:

\(\displaystyle m \angle B < m \angle C < m \angle A\)

Explanation:

\(\displaystyle m \angle A = 66^{\circ }\)

Corresponding angles of similar triangles are congruent, so \(\displaystyle m \angle C =m \angle F = 63^{\circ }\).

Consequently, 

\(\displaystyle m \angle B = 180^{\circ } - (m \angle A +m \angle C)\)

\(\displaystyle m \angle B = 180^{\circ } - ( 66^{\circ }+ 63^{\circ })\)

\(\displaystyle m \angle B = 180^{\circ } - 129^{\circ } = 51^{\circ }\)

Therefore,

\(\displaystyle m \angle B < m \angle C < m \angle A\).

Example Question #3 : How To Find If Two Acute / Obtuse Triangles Are Similar

\(\displaystyle \bigtriangleup ABC \sim \bigtriangleup DEF\)\(\displaystyle AB = 50, BC = DE= 75, DF = 100\)

Which of the following correctly gives the relationship of the angles of \(\displaystyle \bigtriangleup ABC\)

Possible Answers:

\(\displaystyle m \angle C< m \angle A < m \angle B\)

\(\displaystyle m \angle A < m \angle B < m \angle C\)

\(\displaystyle m \angle B < m \angle C < m \angle A\)

\(\displaystyle m \angle C < m \angle B < m \angle A\)

\(\displaystyle m \angle B < m \angle A < m \angle C\)

Correct answer:

\(\displaystyle m \angle C < m \angle B < m \angle A\)

Explanation:

Corresponding sides of similar triangles are in proportion; since \(\displaystyle \bigtriangleup ABC \sim \bigtriangleup DEF\),

\(\displaystyle \frac{AC}{DF} = \frac{AB}{DE}\)

\(\displaystyle \frac{AC}{100} = \frac{50}{75}\)

\(\displaystyle \frac{AC}{100} \cdot 100 = \frac{50}{75} \cdot 100\)

\(\displaystyle AC = 66\frac{2}{3}\)

Therefore, \(\displaystyle AB < AC < BC\)

The angle opposite the longest (shortest) side of a triangle is the angle of greatest (least) measure, so

\(\displaystyle m \angle C < m \angle B < m \angle A\).

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