Question 258859
The Area of a circle is {{{3.14(Pi) * R^2}}}
The Area of the circular ring is the difference of the area of the larger circle minus the area of the inner circle.

Given: Radius of the outer circle is 7.  The difference of the areas between the two circles is 90

Our Equation:  {{{3.14*7^2 - 3.14*A^2 = 90}}}
The left side of the equation can be simplified by dividing both terms by 3.14
{{{3.14 * (7^2 - A^2) = 90}}}
Divide both sides by 3.14
{{{7^2 - A^2 = 28.65}}}
Simplify the equation
{{{49 - A^2 = 28.65}}}
Add A^2 to both sides
{{{49 = 28.65 + A^2}}}
Subtract 28.65 from both sides
{{{20.35 = A^2}}}
Take the square root of both sides
{{{highlight(4.5 = A)}}}

The radius of the inner ring is 4.5