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z^2-7z-9=0
a = 1; b = -7; c = -9;
Δ = b2-4ac
Δ = -72-4·1·(-9)
Δ = 85
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:$z_{1}=\frac{-b-\sqrt{\Delta}}{2a}$$z_{2}=\frac{-b+\sqrt{\Delta}}{2a}$$z_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-7)-\sqrt{85}}{2*1}=\frac{7-\sqrt{85}}{2} $$z_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-7)+\sqrt{85}}{2*1}=\frac{7+\sqrt{85}}{2} $
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