Question

if the area of circumference of circle whose area is 4 times the area of circle with the diameter is 21 CM

Answer 1

area of circle with diameter 21cm is πr²
(22/7)×(21/2)² = 693/2 = 346.5 cm²
this area is 4 times more so 346.5×4 = 1386
now πr² = 1386 (this is first circle area)
r² = (1386×7)/22 = 441
r = 21 then circumference = 2πr = 2×(22/7)× 21
= 132cm

Answer 2

Answer:

42$\pi$

Step-by-step explanation:

Area of a circle with diameter 21 cm: $(\frac{21}{2})^{2} * \pi$ = $\frac{21^{2} }{4} *\pi$

Area of the circle whose area is  4 times the area of a circle with diameter 21 cm: $\frac{21^{2} }{4} *\pi *4 = 21^{2} *\pi$ => Radius of the circle is 21cm

=> Circumference of that circle: 21*2*$\pi$ = 42$\pi$