NY Next Generation Math NY-3.MD.8.a

Math3rd GradeGeometric measurement: recognize perimeter as an attribute of plane figures and distinguish between linear and area measures.

The Standard

Solve real world and mathematical problems involving perimeters of polygons, including finding the perimeter given the side lengths or finding one unknown side length given the perimeter and other side lengths.

New York State Next Generation Mathematics Learning Standards

Teacher's field guide

What This Standard Means

What Students Need to Do

Students solve perimeter problems for polygons using side lengths. They add all sides or find one unknown side from the total perimeter and known sides.

What Mastery Looks Like

A student finds a polygon's perimeter by summing every side once. Given perimeter 30 units and known sides totaling 22, the student finds the missing side as 8 units.

Common Misconceptions

Students may omit a side, count a shared interior segment, or multiply two dimensions as though finding area. They may subtract the unknown from a single side instead of the total known lengths.

How to Assess It

Give a quadrilateral with perimeter 38 centimeters and three sides 9, 11, and 8 centimeters. Ask for the missing side and a checking equation.

Lesson moves

Ways to Teach It

  1. Trace polygon edges with string, record each side length, and add the complete boundary for perimeter.

  2. Ask students how a known perimeter can help find one missing side.

  3. Play perimeter puzzle match with polygon diagrams, known side sums, missing lengths, and totals.

  4. Find a missing fence length when the total boundary and other side lengths are known.

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