5r^2-1=8r

Solution for 5r^2-1=8r equation:

5r^2-1=8r

We move all terms to the left:

5r^2-1-(8r)=0

a = 5; b = -8; c = -1;
Δ = b2-4ac
Δ = -82-4·5·(-1)
Δ = 84
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{84}=\sqrt{4*21}=\sqrt{4}*\sqrt{21}=2\sqrt{21}$
$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-8)-2\sqrt{21}}{2*5}=\frac{8-2\sqrt{21}}{10}
$
$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-8)+2\sqrt{21}}{2*5}=\frac{8+2\sqrt{21}}{10}
$