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20=n(n-1)
We move all terms to the left:
20-(n(n-1))=0
We calculate terms in parentheses: -(n(n-1)), so:We get rid of parentheses
n(n-1)
We multiply parentheses
n^2-1n
Back to the equation:
-(n^2-1n)
-n^2+1n+20=0
We add all the numbers together, and all the variables
-1n^2+n+20=0
a = -1; b = 1; c = +20;
Δ = b2-4ac
Δ = 12-4·(-1)·20
Δ = 81
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:$n_{1}=\frac{-b-\sqrt{\Delta}}{2a}$$n_{2}=\frac{-b+\sqrt{\Delta}}{2a}$$\sqrt{\Delta}=\sqrt{81}=9$$n_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(1)-9}{2*-1}=\frac{-10}{-2} =+5 $$n_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(1)+9}{2*-1}=\frac{8}{-2} =-4 $
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