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2x^2+21x-81=0
a = 2; b = 21; c = -81;
Δ = b2-4ac
Δ = 212-4·2·(-81)
Δ = 1089
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}$$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$$\sqrt{\Delta}=\sqrt{1089}=33$$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(21)-33}{2*2}=\frac{-54}{4} =-13+1/2 $$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(21)+33}{2*2}=\frac{12}{4} =3 $
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