10r2=400r

Solution for 10r2=400r equation:

10r^2=400r

We move all terms to the left:

10r^2-(400r)=0

a = 10; b = -400; c = 0;
Δ = b2-4ac
Δ = -4002-4·10·0
Δ = 160000
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{160000}=400$
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-400)-400}{2*10}=\frac{0}{20}
=0
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-400)+400}{2*10}=\frac{800}{20}
=40
$