Virginia SOL 5.MG.3.g
The Standard
Use models to prove that the sum of the interior angles of a triangle is 180 degrees and use the relationship to determine an unknown angle measure in a triangle.
Virginia Standards of Learning for Mathematics · The student will classify and measure angles and triangles, and solve problems, including those in context.
Teacher's field guide
What This Standard Means
What Students Need to Do
- Students use paper triangles, drawings, or angle pieces to show why the three interior angles combine to make a straight angle. They use that relationship to calculate a missing angle.
What Mastery Looks Like
- Students can rearrange the three corners of a paper triangle to make a straight angle. They can write an equation and accurately find a missing angle from two known measures.
Common Misconceptions
- Students may assume all three angles are equal or add the two known angles without subtracting from 180. They may also confuse interior angles with exterior angles.
How to Assess It
- Give students a triangle with angles labeled 48°, 67°, and x°. Ask them to find x and explain why their answer is correct with an equation or sketch.
Lesson moves
Ways to Teach It
Have students tear off the corners of paper triangles and place the three angles together along a ruler to form a straight angle.
Ask students to explain in writing why a triangle cannot have two right angles, using angle measures as evidence.
Play Missing Angle Match by pairing triangle cards showing two angles with cards showing the correct third angle.
Use a roof truss diagram with two labeled angles and have students calculate the missing angle where the beams meet.
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Related Standards
- 8.MG.1.b
Use the relationships among supplementary, complementary, vertical, and adjacent angles to solve problems, including those in context, involving the measure of ...
- 7.MG.2.g
Given angle measures in a quadrilateral or triangle, determine unknown angle measures in a similar quadrilateral or triangle.
- 5.MG.3.h
Solve addition and subtraction contextual problems to determine unknown angle measures on a diagram.
- 8.MG.4.d
Determine the measure of a side of a right triangle, given the measures of the other two sides.
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