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(5x-9)(4x+9)=0
We multiply parentheses ..
(+20x^2+45x-36x-81)=0
We get rid of parentheses
20x^2+45x-36x-81=0
We add all the numbers together, and all the variables
20x^2+9x-81=0
a = 20; b = 9; c = -81;
Δ = b2-4ac
Δ = 92-4·20·(-81)
Δ = 6561
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{6561}=81$$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(9)-81}{2*20}=\frac{-90}{40} =-2+1/4 $$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(9)+81}{2*20}=\frac{72}{40} =1+4/5 $
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