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9x^2=-21x
We move all terms to the left:
9x^2-(-21x)=0
We get rid of parentheses
9x^2+21x=0
a = 9; b = 21; c = 0;
Δ = b2-4ac
Δ = 212-4·9·0
Δ = 441
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{441}=21$$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(21)-21}{2*9}=\frac{-42}{18} =-2+1/3 $$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(21)+21}{2*9}=\frac{0}{18} =0 $
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