8th grade · Math · Unit 8 of 17
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.
Alabama Course of Study content standard: 8.25 · angle sums of triangles and angles formed by parallel lines cut by 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 Alabama
In the Alabama Course of Study, this is unit 8 of 17 in 8th grade Math, after Tessellations & Transformations and before Tables, Graphs & Linear Relations.
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.
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
A transversal crosses two parallel lines. One angle measures 48°. How many degrees is a vertically opposite angle to it?
answer box°
48 °
Hint: These angles are equal when the lines are parallel.
Question 2
Two co-interior angles between parallel lines measure (x + 29)° and (3x + 31)°. What is x?
answer box
30
Hint: Co-interior angles add to 180°. Solve x + 29 + 3x + 31 = 180.
Question 3
In a triangle, two interior angles are 43° and 53°. What is the exterior angle at the third vertex?
answer box°
96 °
Hint: An exterior angle equals the sum of the two opposite interior angles: 43 + 53.
Question 4
What is the sum of the interior angles of a polygon with 12 sides?
answer box°
1800 °
Hint: Split it into triangles: (n − 2) × 180° = 10 × 180°.
Question 5
Three angles of a quadrilateral are 70°, 94° and 101°. What is the fourth angle?
answer box°
95 °
Hint: The angles of a quadrilateral add to 360°. Subtract the three you know from 360°.
Question 6
What is the measure of each interior angle of a regular polygon with 5 sides?
answer box°
108 °
Hint: Find the sum, 540°, then divide by 5.