Semicircles: calculate area, perimeter, radius and diameter

key notes :

Key Definitions

• Perimeter: The total distance around a closed shape (boundary length).
• Area: The surface covered by a closed shape (space inside).
• Radius (r): Distance from the center of a circle to its boundary.
• Diameter (d): Longest chord of a circle, passing through the center. d=2r and r=d/2​

Formulas

(A) Rectangle

• Area: A=L×B
• Perimeter: P=2(L+B)

(B) Square

• Area: A=side²
• Perimeter: P=4×side

(C) Triangle

• Area: A=1/2×base×height
• Perimeter: P=a+b+c

(D) Circle

• Circumference (Perimeter): C=2πr or C=πd
• Area: A=πr²

Examples

Example 1: Rectangle

Length = 12 cm, Breadth = 8 cm

• Area = 12×8=96 cm²
• Perimeter = 2(12+8)=40 cm

Example 2: Square

Side = 15 m

• Area = 15²=225 m²
• Perimeter = 4×15=60 m

Example 3: Triangle

Base = 10 cm, Height = 8 cm, Other two sides = 7 cm and 9 cm

• Area = 1/2×10×8=40 cm²
• Perimeter = 10+7+9=26 cm

Example 4: Circle

Radius = 7 cm

• Diameter = 2×7=14 cm
• Circumference = 2×227×7=44 cm
• Area = 227×7×7=154 cm²

Important Points

• Perimeter is always in linear units (cm, m, km).
• Area is always in square units (cm², m², km²).
• For circles, check if radius or diameter is given.
• Use π=3.14 or 22/7.

Practice Questions

1. A rectangle has length 18 cm and breadth 12 cm. Find its area and perimeter.
2. The side of a square is 25 m. Find its area and perimeter.
3. A triangle has base 15 cm, height 12 cm, and the other two sides are 10 cm and 13 cm. Find its area and perimeter.
4. A circular park has a diameter of 28 m. Find its radius, circumference, and area.
5. The circumference of a circle is 62.8 cm. Find its radius and area.

Learn with an example

Let’s practice!🖊️