Understanding Elementary Shapes (NCERT Ch. 5)

• Measure and compare line segments.
• Classify angles: acute (<90°), right (=90°), obtuse (90°-180°), straight (=180°), reflex (>180°).
• Classify triangles by sides (scalene, isosceles, equilateral) and angles (acute, right, obtuse).
• Classify quadrilaterals (square, rectangle, rhombus, trapezium, parallelogram) and identify polygons and basic 3D shapes (cube, cuboid, sphere, cylinder, cone, pyramid).

1. Acute angle — measures less than 90°.
2. Right angle — measures exactly 90°.
3. Obtuse angle — measures more than 90° but less than 180°.
4. Straight angle — measures exactly 180°.
5. Reflex angle — measures more than 180° but less than 360°.
6. Equilateral triangle — all 3 sides equal, all angles 60°.
7. Isosceles triangle — exactly 2 sides equal.
8. Scalene triangle — no sides equal.
9. 3D shapes: cube (6 square faces), cuboid (6 rectangular faces), sphere, cylinder, cone, pyramid.

Classify an angle that measures 120°.

90° < 120° < 180°, so it is an obtuse angle.

• Mixing up obtuse (between 90° and 180°) with reflex (over 180°).
• Assuming all triangles with two equal-looking sides are equilateral (check all three).
• Confusing a cube (all square faces) with a cuboid (rectangular faces).

• Name the type of triangle with sides 5 cm, 5 cm, 5 cm.
• What do you call an angle greater than 180°?

Open the Practice tab for graded questions on Understanding Elementary Shapes (NCERT Ch. 5).