9=3.14((21)2-r2)

Solution for 9=3.14((21)2-r2) equation:

9=3.14((21)2-r2)

We move all terms to the left:

9-(3.14((21)2-r2))=0

We add all the numbers together, and all the variables

-(3.14(-1r^2+212))+9=0


We calculate terms in parentheses: -(3.14(-1r^2+212)), so:

3.14(-1r^2+212)

We multiply parentheses

-3.14r^2+665.68

Back to the equation:

-(-3.14r^2+665.68)


We get rid of parentheses

3.14r^2-665.68+9=0

We add all the numbers together, and all the variables

3.14r^2-656.68=0

a = 3.14; b = 0; c = -656.68;
Δ = b2-4ac
Δ = 02-4·3.14·(-656.68)
Δ = 8247.9008
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-\sqrt{8247.9008}}{2*3.14}=\frac{0-\sqrt{8247.9008}}{6.28}
=-\frac{\sqrt{}}{6.28}
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+\sqrt{8247.9008}}{2*3.14}=\frac{0+\sqrt{8247.9008}}{6.28}
=\frac{\sqrt{}}{6.28}
$