ISEE Upper Level Quantitative : How to find an angle

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

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Example Questions

Example Question #11 : Geometry

Obtuse

Refer to the above figure. Which is the greater quantity?

(a) \displaystyle x+ y

(b) \displaystyle z

Possible Answers:

(a) and (b) are equal

(b) is the greater quantity

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

(a) is the greater quantity

Correct answer:

(a) is the greater quantity

Explanation:

Extend \displaystyle \overline{AB} as seen in the figure below:

Obtuse

The measure of an exterior angle of a triangle is equal to the sum of the measures of its remote interior angles; specifically,

\displaystyle m \angle CAD = m \angle B + m \angle C,

and

 \displaystyle m \angle CAD =( x+ y) ^{\circ }

However, \displaystyle m \angle CAD > z ^{\circ }, so, by substitution,

\displaystyle x+y>z

Example Question #11 : Plane Geometry

Given: \displaystyle \bigtriangleup ABC\displaystyle AB= AC = 10, BC = 12. Which is the greater quantity?

(a) \displaystyle m \angle B

(b) \displaystyle 60^{\circ }

Possible Answers:

(a) and (b) are equal

(b) is the greater quantity

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

(a) is the greater quantity

Correct answer:

(b) is the greater quantity

Explanation:

Below is the referenced triangle along with \displaystyle \bigtriangleup DEF, an equilateral triangle with sides of length 10:

Triangles

As an angle of an equilateral triangle, \displaystyle \angle D has measure \displaystyle 60 ^{\circ }. Applying the Side-Side-Side Inequality Theorem, since \displaystyle AB = DE\displaystyle AC = DF, and \displaystyle BC > EF, it follows that \displaystyle m \angle A > m \angle D, so \displaystyle m \angle A > 60^{\circ }.

Also, since \displaystyle AB = BC, by the Isosceles Triangle Theorem, \displaystyle m \angle B = m \angle C. Since \displaystyle m \angle A > 60^{\circ }, and the sum of the measures of the angles of a triangle is \displaystyle 180 ^{\circ }, it follows that

\displaystyle m \angle B + m \angle C < 120^{\circ }

Substituting and solving:

\displaystyle m \angle B + m \angle B < 120^{\circ }

\displaystyle 2 m \angle B < 120^{\circ }

\displaystyle 2 m \angle B \div 2 < 120^{\circ } \div 2

\displaystyle m \angle B < 60^{\circ }.

 

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