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x=-0.4x^2+9.6x+15
We move all terms to the left:
x-(-0.4x^2+9.6x+15)=0
We get rid of parentheses
0.4x^2-9.6x+x-15=0
We add all the numbers together, and all the variables
0.4x^2-8.6x-15=0
a = 0.4; b = -8.6; c = -15;
Δ = b2-4ac
Δ = -8.62-4·0.4·(-15)
Δ = 97.96
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{-(-8.6)-\sqrt{97.96}}{2*0.4}=\frac{8.6-\sqrt{97.96}}{0.8} $$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-8.6)+\sqrt{97.96}}{2*0.4}=\frac{8.6+\sqrt{97.96}}{0.8} $
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