8th grade · Math · Unit 12 of 15
Angle Detective
Parallel lines and polygons
What your child practices
Using equal and supplementary angles when a transversal crosses parallel lines, and the angle sums of triangles, quadrilaterals and polygons, including exterior angles.
Texas Essential Knowledge and Skills (TEKS) teks student expectation: 8.8(D) · angle sums of triangles and angles formed by parallel lines and a transversal
How we explain it
Children can open this short lesson before they practice.
- Angles on a straight line add to 180°.
- The angles in a triangle add to 180°.
- When two lines cross, opposite angles are equal.
Example: Two angles on a straight line: one is 115°. What is the other? Together they make 180°. 180 − 115 = 65. Answer: 65°
Angle Detective in Texas
In the Texas Essential Knowledge and Skills (TEKS), this is unit 12 of 15 in 8th grade Math, after Pythagorean Theorem and before Systems of Equations.
Gradelings describes a child's practice on this unit as Beginning, Approaching, Meeting or Exceeding (Four practice steps). That reflects practice, not a report-card mark: the teacher decides the mark.
Source: Texas Education Agency (TEA)
Sample questions
A taste of this unit. In the app, questions change every time and get easier or harder to match your child.
Question 1
What is the measure of each exterior angle of a regular polygon with 12 sides?
answer box°
30 °
Hint: The exterior angles of any polygon add to 360°. Divide 360° by 12.
Question 2
What is the measure of each interior angle of a regular polygon with 8 sides?
answer box°
135 °
Hint: Find the sum, 1080°, then divide by 8.
Question 3
Three angles of a quadrilateral are 73°, 104° and 82°. What is the fourth angle?
answer box°
101 °
Hint: The angles of a quadrilateral add to 360°. Subtract the three you know from 360°.
Question 4
A transversal crosses two parallel lines. One angle measures 75°. How many degrees is a vertically opposite angle to it?
answer box°
75 °
Hint: These angles are equal when the lines are parallel.
Question 5
In a triangle, two interior angles are 44° and 46°. What is the exterior angle at the third vertex?
answer box°
90 °
Hint: An exterior angle equals the sum of the two opposite interior angles: 44 + 46.
Question 6
Two co-interior angles between parallel lines measure (x + 28)° and (3x + 36)°. What is x?
answer box
29
Hint: Co-interior angles add to 180°. Solve x + 28 + 3x + 36 = 180.