ISEE Upper Level Math : How to find if right triangles are similar

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

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Example Questions

Example Question #1 : Right Triangles

\(\displaystyle \Delta ABC \sim \Delta DEF\)

\(\displaystyle \angle B\) is a right angle; \(\displaystyle DE = 21\)\(\displaystyle DF = 42\).

Which is the greater quantity?

(a) \(\displaystyle m \angle A\)

(b) \(\displaystyle 60 ^{\circ }\)

Possible Answers:

(b) is greater.

(a) is greater.

It is impossible to tell from the information given.

(a) and (b) are equal.

Correct answer:

(a) and (b) are equal.

Explanation:

\(\displaystyle \Delta ABC \sim \Delta DEF\). Corresponding angles of similar triangles are congruent, so since \(\displaystyle \angle B\) is a right angle, so is \(\displaystyle \angle E\)

The hypotenuse \(\displaystyle \overline{DF}\) of \(\displaystyle \Delta DEF\) is twice as long as leg \(\displaystyle \overline{DE}\); by the \(\displaystyle 30 ^{\circ } - 60^{\circ } -90^{\circ }\) Theorem, \(\displaystyle m \angle D = 60 ^{\circ }\). Again, by similiarity, 

\(\displaystyle m \angle A = m \angle D = 60 ^{\circ }\).

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