Take a rectangular paper of length (l) and breadth (b). So, area of rectangle is l × b square unit.

Draw a diagonal in the rectangle as shown in the figure. Fold the paper and cut the folded part. Now, the rectangle is divided into two right angled triangles.

Area of rectangle = 2 × Area of right angled triangles

∴ Area of right-angled triangle = 1/2 × Area of rectangle

Take one rectangular squared paper in pairs and make one triangle of equal size as shown in the figure. Now, what is the area of that triangle? Discuss in pairs.

From the first piece, Area of a triangle = 1/2 × Area of rectangular paper (after cutting) = 1/2 × 4 × 5 = 10 square units

Similarly, from the second piece, Area of a triangle = 1/2 × 2 × 5 = 5 square units

So, total area of triangle = 10 + 5 = 15 square units

Take a rectangular paper of certain size. Let, the base be (b) and height be (h). Then,

Area of rectangle = base × height = b × h square units

Therefore, Area of triangle = 1/2 × Area of rectangle = 1/2 × b × h