84.78=3.14r^2

Solution for 84.78=3.14r^2 equation:

84.78=3.14r^2

We move all terms to the left:

84.78-(3.14r^2)=0

We get rid of parentheses

-3.14r^2+84.78=0

a = -3.14; b = 0; c = +84.78;
Δ = b2-4ac
Δ = 02-4·(-3.14)·84.78
Δ = 1064.8368
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{1064.8368}}{2*-3.14}=\frac{0-\sqrt{1064.8368}}{-6.28}
=-\frac{\sqrt{}}{-6.28}
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+\sqrt{1064.8368}}{2*-3.14}=\frac{0+\sqrt{1064.8368}}{-6.28}
=\frac{\sqrt{}}{-6.28}
$