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r2-21r+80=0
We add all the numbers together, and all the variables
r^2-21r+80=0
a = 1; b = -21; c = +80;
Δ = b2-4ac
Δ = -212-4·1·80
Δ = 121
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{121}=11$$r_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-21)-11}{2*1}=\frac{10}{2} =5 $$r_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-21)+11}{2*1}=\frac{32}{2} =16 $
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