Question

find area of circle whose diameter =21 cm​

Answer 1

d = 21

C = 66

A = πr2 = π(d

2

)2

A = C2

4π

π = 3.1415

A = area

C = circumference or perimeter

r = radius, d = diameter

Solution Steps:

Area of a circle in terms of radius:

Area = π·r2 = 3.14·10.52 = 346.4 square cm(*)

Area of a circle in terms of diameter:

Area = π·(d

2

)2 = 3.14·(21

2

)2 = 3.14·(10.5)2 = 346.4 square cm(*)

Area of a circle in terms of circumference:

Area = C2

4π

= 65.972

4π

= 4352.04

(4·3.14)

= 4352.04

12.56

= 346.4 square cm(*)

(*) 346.36059005827 cm, exactly or limited to the precision of this calculator (13 decimal places).

Note: for simplicity, the operations above were rounded to 2 decimal places and π was rounded to 3.14.

Answer 2

Answer:

Area of circle =

$\pi {r}^{2}$

r=d÷2

21÷2

=11.5cm

415.57