SAT II Math II : Finding Angles

Study concepts, example questions & explanations for SAT II Math II

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Example Questions

Example Question #3 : Linear Functions

Solve for \(\displaystyle x\) and \(\displaystyle y\).

Question_3

(Figure not drawn to scale).

Possible Answers:

\(\displaystyle \small x=15^o;\ y=7.5^o\)

\(\displaystyle \small x=15^o;\ y=37.5^o\)

\(\displaystyle \small x=7.5^o;\ y=18.75^o\)

\(\displaystyle \small x=15^o;\ y=52.5^o\)

Correct answer:

\(\displaystyle \small x=15^o;\ y=37.5^o\)

Explanation:

The angles containing the variable \(\displaystyle x\) all reside along one line, therefore, their sum must be \(\displaystyle 180^o\).

\(\displaystyle \small 5x+4x+3x=180^o\)

\(\displaystyle \small 12x=180^o\)

\(\displaystyle \small x=15^o\)

Because \(\displaystyle 2y\) and \(\displaystyle 5x\) are opposite angles, they must be equal.

\(\displaystyle \small 2y=5x\)

\(\displaystyle \small x=15^o\)

\(\displaystyle \small 2y=5(15^o)=75^o\)

\(\displaystyle \small y=\frac{75^o}{2}=37.5^o\)

Example Question #1 : Finding Angles

What angle do the minute and hour hands of a clock form at 6:15?

Possible Answers:

\(\displaystyle 92 \frac{1}{2}^{\circ }\)

\(\displaystyle 95^{\circ }\)

\(\displaystyle 90^{\circ }\)

\(\displaystyle 97 \frac{1}{2}^{\circ }\)

\(\displaystyle 87 \frac{1}{2}^{\circ }\)

Correct answer:

\(\displaystyle 97 \frac{1}{2}^{\circ }\)

Explanation:

There are twelve numbers on a clock; from one to the next, a hand rotates \(\displaystyle \left (\frac{360}{12} \right )^{\circ } = 30^{\circ }\). At 6:15, the minute hand is exactly on the "3" - that is, on the \(\displaystyle \left (30 \times 3 \right )^{\circ }= 90^{\circ }\) position. The hour hand is one-fourth of the way from the "6" to the "7" - that is, on the \(\displaystyle \left (6 \frac{1}{4} \times 30 \right )^{\circ } = 187 \frac{1}{2} ^{\circ }\) position. Therefore, the difference is the angle they make:

\(\displaystyle 187 \frac{1}{2} ^{\circ } - 90^{\circ } = 97 \frac{1}{2}^{\circ }\).

Example Question #2 : Finding Angles

In triangle \(\displaystyle \Delta ABC\)\(\displaystyle m\angle A = 50^{\circ }\) and \(\displaystyle m \angle B = 80^{\circ }\). Which of the following describes the triangle?

Possible Answers:

None of the other responses is correct.

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

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

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

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

Correct answer:

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

Explanation:

Since the measures of the three interior angles of a triangle must total \(\displaystyle 180^{\circ }\)

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

\(\displaystyle m \angle C + 50^{\circ } + 80^{\circ } = 180^{\circ }\)

\(\displaystyle m \angle C + 130^{\circ } = 180^{\circ }\)

\(\displaystyle m \angle C = 50^{\circ }\)

All three angles have measure less than \(\displaystyle 90^{\circ }\), making the triangle acute. Also, by the Isosceles Triangle Theorem, since \(\displaystyle m \angle A= m \angle C = 50^{\circ }\)\(\displaystyle BC = AB\); the triangle has two congruent sides and is isosceles. 

Example Question #3 : Finding Angles

In \(\displaystyle \Delta DEF\)\(\displaystyle \angle D\) and \(\displaystyle \angle E\) are complementary, and \(\displaystyle m\angle D > m \angle E\). Which of the following is true of \(\displaystyle \Delta DEF\) ?

Possible Answers:

None of the other responses is correct.

\(\displaystyle \Delta DEF\) is acute and scalene.

\(\displaystyle \Delta DEF\) is acute and isosceles.

\(\displaystyle \Delta DEF\) is right and scalene.

\(\displaystyle \Delta DEF\) is right and isosceles.

Correct answer:

\(\displaystyle \Delta DEF\) is right and scalene.

Explanation:

\(\displaystyle \angle D\) and \(\displaystyle \angle E\) are complementary, so, by definition, \(\displaystyle m\angle D + m\angle E = 90^{\circ }\)

Since the measures of the three interior angles of a triangle must total \(\displaystyle 180^{\circ }\)

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

\(\displaystyle m \angle F + 90^{\circ } = 180^{\circ }\)

\(\displaystyle m \angle F = 90^{\circ }\)

\(\displaystyle \angle F\) is a right angle, so \(\displaystyle \Delta DEF\) is a right triangle. 

\(\displaystyle \angle D\) and \(\displaystyle \angle E\) must be acute, so neither is congruent to \(\displaystyle \angle F\); also, \(\displaystyle \angle D\) and \(\displaystyle \angle E\)  are not congruent to each other. Therefore, all three angles have different measure. Consequently, all three sides have different measure, and \(\displaystyle \Delta DEF\) is scalene.

Example Question #4 : Finding Angles

Decagon

The above figure is a regular decagon. Evaluate \(\displaystyle m \angle ACB\).

Possible Answers:

\(\displaystyle 18^{\circ }\)

\(\displaystyle 15^{\circ }\)

\(\displaystyle 12^{\circ }\)

\(\displaystyle 20^{\circ }\)

\(\displaystyle 24^{\circ }\)

Correct answer:

\(\displaystyle 18^{\circ }\)

Explanation:

As an interior angle of a regular decagon, \(\displaystyle \angle B\) measures

\(\displaystyle m \angle B = \left [\frac{180(10-2)}{10} \right ] ^{\circ }= 144^{\circ }\).

Since \(\displaystyle \overline{AB}\) and \(\displaystyle \overline{BC}\) are two sides of a regular polygon, they are congruent. Therefore, by the Isosceles Triangle Theorem,

\(\displaystyle m \angle ACB = m \angle CAB\)

The sum of the measures of a triangle is \(\displaystyle 180^{\circ }\), so

\(\displaystyle m \angle ACB + m \angle CAB + m \angle ABC =180^{\circ }\)

\(\displaystyle m \angle ACB + m \angle ACB + 144 ^{\circ } =180^{\circ }\)

\(\displaystyle 2 m \angle ACB + 144 ^{\circ } =180^{\circ }\)

\(\displaystyle 2 m \angle ACB =36^{\circ }\)

\(\displaystyle m \angle ACB =18^{\circ }\)

Example Question #5 : Finding Angles

Hexagon

The above hexagon is regular. What is \(\displaystyle x\)?

Possible Answers:

\(\displaystyle x = 69\)

\(\displaystyle x = 79\)

\(\displaystyle x = 64\)

\(\displaystyle x = 74\)

None of the other responses is correct.

Correct answer:

\(\displaystyle x = 79\)

Explanation:

Two of the angles of the quadrilateral formed are angles of a regular hexagon, so each measures

\(\displaystyle \frac{180^{\circ } (6-2)}{6}= 120^{\circ }\).

The four angles of the quadrilateral are \(\displaystyle 120 ^{\circ }, 120^{\circ }, 41^{\circ }, x^{\circ }\). Their sum is \(\displaystyle 360^{\circ }\), so we can set up, and solve for \(\displaystyle x\) in, the equation:

\(\displaystyle x + 120 + 120 + 41 =360\)

\(\displaystyle x + 281 =360\)

\(\displaystyle x = 79\)

Example Question #61 : 2 Dimensional Geometry

What angle do the minute and hour hands of a clock form at 4:15?

Possible Answers:

\(\displaystyle 30 ^{\circ }\)

\(\displaystyle 40 ^{\circ }\)

\(\displaystyle 37\frac{1}{2} ^{\circ }\)

\(\displaystyle 32\frac{1}{2} ^{\circ }\)

\(\displaystyle 27\frac{1}{2} ^{\circ }\)

Correct answer:

\(\displaystyle 37\frac{1}{2} ^{\circ }\)

Explanation:

There are twelve numbers on a clock; from one to the next, a hand rotates \(\displaystyle \left (\frac{360}{12} \right )^{\circ } = 30^{\circ }\). At 4:15, the minute hand is exactly on the "3" - that is, on the \(\displaystyle \left (30 \times 3 \right )^{\circ }= 90^{\circ }\) position. The hour hand is one-fourth of the way from the "4" to the "5" - that is, on the \(\displaystyle \left (4 \frac{1}{4} \times 30 \right )^{\circ } = 127 \frac{1}{2} ^{\circ }\) position. Therefore, the difference is the angle they make:

\(\displaystyle 127 \frac{1}{2} ^{\circ } - 90^{\circ } = 37 \frac{1}{2}^{\circ }\).

Example Question #62 : 2 Dimensional Geometry

If the vertical angles of intersecting lines are:  \(\displaystyle 2x-9\) and \(\displaystyle 3x+6\), what must be the value of \(\displaystyle x\)?

Possible Answers:

\(\displaystyle -15\)

\(\displaystyle \frac{183}{5}\)

\(\displaystyle -3\)

\(\displaystyle \frac{93}{5}\)

\(\displaystyle 3\)

Correct answer:

\(\displaystyle -15\)

Explanation:

Vertical angles of intersecting lines are always equal.

Set the two expressions equal to each other and solve for \(\displaystyle x\).

\(\displaystyle 2x-9 = 3x+6\)

Subtract \(\displaystyle 2x\) from both sides.

\(\displaystyle 2x-9 -2x= 3x+6-2x\)

\(\displaystyle -9 = x+6\)

Subtract 6 from both sides.

\(\displaystyle -9 -6= x+6-6\)

The answer is:  \(\displaystyle -15\)

Example Question #63 : 2 Dimensional Geometry

If the angles in degrees are \(\displaystyle x+3\) and \(\displaystyle x+7\) which are complementary to each other, what is three times the value of the smallest angle?

Possible Answers:

\(\displaystyle 129\)

\(\displaystyle 141\)

\(\displaystyle 264\)

\(\displaystyle 255\)

\(\displaystyle 120\)

Correct answer:

\(\displaystyle 129\)

Explanation:

Complementary angles add up to 90 degrees.

Set up an equation such that the sum of both angles equal to 90.

\(\displaystyle x+3+x+7 = 90\)

\(\displaystyle 2x+10 = 90\)

Subtract 10 from both sides.

\(\displaystyle 2x+10 -10= 90-10\)

\(\displaystyle 2x=80\)

Divide by 2 on both sides.

\(\displaystyle \frac{2x}{2}=\frac{80}{2}\)

\(\displaystyle x=40\)

The angles are:

\(\displaystyle x+3 = 40+3 = 43\)

\(\displaystyle x+7 = 40+7 = 47\)

Three times the value of the smallest angle is:

\(\displaystyle 43\times 3 = 129\)

The answer is:  \(\displaystyle 129\)

Example Question #64 : 2 Dimensional Geometry

If the angles \(\displaystyle -10x-5\) and \(\displaystyle 5x\) are supplementary, what must be the value of \(\displaystyle x\)?

Possible Answers:

\(\displaystyle \frac{37}{3}\)

\(\displaystyle 37\)

\(\displaystyle -\frac{37}{3}\)

\(\displaystyle -37\)

\(\displaystyle 19\)

Correct answer:

\(\displaystyle -37\)

Explanation:

Supplementary angles sum up to 180 degrees.

\(\displaystyle -10x-5+5x = 180\)

\(\displaystyle -5x-5 = 180\)

Add five on both sides.

\(\displaystyle -5x-5 +5= 180+5\)

\(\displaystyle -5x = 185\)

Divide by negative five on both sides to determine \(\displaystyle x\).

\(\displaystyle \frac{-5x}{-5} = \frac{185}{-5}\)

\(\displaystyle x=-37\)

The answer is:  \(\displaystyle -37\)

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