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4.2x(15+5x)=-20x-21-1x
We move all terms to the left:
4.2x(15+5x)-(-20x-21-1x)=0
We add all the numbers together, and all the variables
4.2x(5x+15)-(-21x-21)=0
We multiply parentheses
20x^2+60x-(-21x-21)=0
We get rid of parentheses
20x^2+60x+21x+21=0
We add all the numbers together, and all the variables
20x^2+81x+21=0
a = 20; b = 81; c = +21;
Δ = b2-4ac
Δ = 812-4·20·21
Δ = 4881
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{-(81)-\sqrt{4881}}{2*20}=\frac{-81-\sqrt{4881}}{40} $$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(81)+\sqrt{4881}}{2*20}=\frac{-81+\sqrt{4881}}{40} $
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