X2-21x+20=0

Solution for X2-21x+20=0 equation:

X2-21X+20=0

We add all the numbers together, and all the variables

X^2-21X+20=0

a = 1; b = -21; c = +20;
Δ = b2-4ac
Δ = -212-4·1·20
Δ = 361
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{361}=19$
$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-21)-19}{2*1}=\frac{2}{2}
=1
$
$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-21)+19}{2*1}=\frac{40}{2}
=20
$