Question

the radius of circle is 7 cm . find the radius of that circle whose area is double of the area of this circle

Answer 1

Given ,Radius = 7cm. 
We know that ,Area of circle = π r² 
Now , Area of circle = 22/7 * 7² = 22*7 = 154 cm² .
Given another circle radius = 2*154 = 308cm² .
Now ,πr² = 308 r² = 98r = √98 cm.

Answer 2

dude it's simple one answer 7√2
cause when we multiply 7√2×7√2 due of r^2
we get 49 ×2
double of previous value
this trick works on any given radius