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7x^2-7x-66=0
a = 7; b = -7; c = -66;
Δ = b2-4ac
Δ = -72-4·7·(-66)
Δ = 1897
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{-(-7)-\sqrt{1897}}{2*7}=\frac{7-\sqrt{1897}}{14} $$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-7)+\sqrt{1897}}{2*7}=\frac{7+\sqrt{1897}}{14} $
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