NCERT Ganita Prakash, Class 7, textbook pages 241–274 (Part 1)

How to teach Class 7 Maths Chapter 7, A Tale of Three Intersecting Lines, at home

Triangles: types, angle sum and construction, taught with things you already have at home: a 20-minute activity, a game for the week, a 5-day plan and the mistakes to watch for.

angles add to 180°abc

NCERT Class 7 Maths Chapter 7 A Tale of Three Intersecting Lines: what it teaches

A triangle is the simplest closed shape. Your child classifies triangles by sides and angles, tests when three lengths can make a triangle (triangle inequality), proves the angle sum is 180°, and constructs triangles with ruler, compass and protractor.

Why it matters later. Triangles underpin congruence (Part 2), the Pythagoras theorem (Class 8) and trigonometry (Class 10).

See it: this chapter in pictures

3 + 4 < 8: gap!5 + 6 > 7 ✓
The two shorter sides must be longer together than the longest.
acba + b + c = 180°
Tear off the corners: they make a straight line.
sides: equilateral, isosceles, scaleneangles: acute, right, obtusean isosceles right triangle exists!
Classify by sides and by angles.

You need

  • straws or sticks of different lengths
  • paper, scissors
  • ruler, compass, protractor

Tonight’s 20 minutes

  1. Try making triangles with sticks of 3, 4, 8 cm. Why doesn’t it close?
  2. Cut out a triangle, tear off the three corners and place them together: they form a straight line.
  3. Construct a triangle with sides 5, 6, 7 cm using a compass.
  4. Sort triangles: equilateral, isosceles, scalene; acute, right, obtuse.

Words to use

  • equilateral
  • isosceles
  • scalene
  • acute
  • obtuse
  • right
  • triangle inequality
  • angle sum

Watch for

  • Thinking any three lengths make a triangle. The two shorter sides must add to more than the longest.
  • Protractor errors when constructing. Mark the angle point lightly, then draw the arm with a ruler.

What it sounds like at home

A short example of the conversation. Your words will differ; the questions are what matter.

YouTwo angles 50° and 60°. The third?

Your child180 − 110 = 70°.

A game for this week: Can it close?

  1. Write stick lengths on cards.
  2. Draw three cards.
  3. Predict: triangle or not, then test with sticks.
  4. Correct predictions score.

A 5-day plan, 20 minutes a day

  1. Day 1Triangle inequality with sticks.
  2. Day 2Angle sum by tearing corners.
  3. Day 3Construct with a compass.
  4. Day 4Classify triangles.
  5. Day 5Can it close? game.

Everyday moments to practise

  • Roof trusses and bridges.
  • Set squares.
  • Samosas and sandwiches.
  • Tent shapes.

Quick check

Ask these at the end of the week. Tap to see the answer.

Can 3, 4 and 8 cm make a triangle?

No: 3 + 4 < 8.

Two angles are 50° and 60°. Third?

70°.

A triangle with two equal sides is called?

Isosceles.

Ready for the next chapter when your child…

  • Uses the triangle inequality.
  • Knows angles sum to 180°.
  • Constructs triangles from given data.

Activities, wording and illustrations are Mathify Kids’ own, following the topics of NCERT’s Ganita Prakash (Class 7). See the full Class 7 parent guide for every chapter.

Questions parents ask

What is the triangle inequality?

Any two sides of a triangle add to more than the third.

Why do triangle angles add to 180°?

Placing the three angles together makes a straight line.

Which constructions are in Class 7?

Triangles from three sides, and from sides with angles.