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x^2-9x-21=0
a = 1; b = -9; c = -21;
Δ = b2-4ac
Δ = -92-4·1·(-21)
Δ = 165
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}$$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-9)-\sqrt{165}}{2*1}=\frac{9-\sqrt{165}}{2} $$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-9)+\sqrt{165}}{2*1}=\frac{9+\sqrt{165}}{2} $
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