ISEE Upper Level Quantitative : How to find if triangles are similar

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

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Example Questions

Example Question #1 : How To Find If Triangles Are Similar

\displaystyle \Delta ABC and \displaystyle \Delta DEF are right triangles, with right angles \displaystyle \angle B , \angle E, respectively. \displaystyle m \angle A = m\angle D = 45^{\circ } and \displaystyle AC > DF.

Which is the greater quantity?

(a) \displaystyle AB

(b) \displaystyle EF

Possible Answers:

(a) is greater.

(a) and (b) are equal.

It is impossible to tell from the information given.

(b) is greater.

Correct answer:

(a) is greater.

Explanation:

Each right triangle is a \displaystyle 45 ^{\circ }-45 ^{\circ }-90 ^{\circ } triangle, making each triangle isosceles by the Converse of the Isosceles Triangle Theorem.

Since \displaystyle \angle B and \displaystyle \angle E are the right triangles, the legs are \displaystyle \overline{AB}, \overline{BC}, \overline{DE}, \overline{EF}, and the hypotenuses are \displaystyle \overline{AC}, \overline{DF}.

By the \displaystyle 45 ^{\circ }-45 ^{\circ }-90 ^{\circ } Theorem, \displaystyle AB \sqrt{2} = AC and \displaystyle EF \sqrt{2} = DF.

\displaystyle AC > DF, so \displaystyle AB \sqrt{2} > EF \sqrt{2} and subsequently, \displaystyle AB > EF.

Example Question #2 : How To Find If Triangles Are Similar

Untitled

Figure NOT drawn to scale.

In the above figure, \displaystyle \angle ABC is a right angle. 

What is the ratio of the area of \displaystyle \bigtriangleup AXB to that of \displaystyle \bigtriangleup BXC ?

Possible Answers:

169 to 25

12 to 5

144 to 25

13 to 5

Correct answer:

144 to 25

Explanation:

The altitude of a right triangle from the vertex of its right angle divides the triangle into two smaller, similar triangles. 

The similarity ratio of \displaystyle \bigtriangleup AXB to \displaystyle \bigtriangleup BXC can be found by determining the ratio of one pair of corresponding sides; we will use the short leg of each, \displaystyle \overline{XB} and \displaystyle \overline{XC}.

\displaystyle \overline{XB} is also the long leg of \displaystyle \bigtriangleup BXC, so its length can be found using the Pythagorean Theorem:

\displaystyle (BX)^{2} = (BC)^{2} - (CX)^{2}

\displaystyle (BX)^{2} = 13^{2} - 5^{2} = 169 - 25 = 144

\displaystyle BX = \sqrt{144} = 12

The similarity ratio is therefore

\displaystyle \frac{XB}{XC} = \frac{12}{5}.

The ratio of the areas is the square of this ratio:

\displaystyle \left ( \frac{12}{5} \right )^{2} = \frac{12^{2}}{5^{2}} = \frac{144}{25} - that is, 144 to 25.

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