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28x+9=21x^2
We move all terms to the left:
28x+9-(21x^2)=0
determiningTheFunctionDomain -21x^2+28x+9=0
a = -21; b = 28; c = +9;
Δ = b2-4ac
Δ = 282-4·(-21)·9
Δ = 1540
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}$$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$
The end solution:
$\sqrt{\Delta}=\sqrt{1540}=\sqrt{4*385}=\sqrt{4}*\sqrt{385}=2\sqrt{385}$$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(28)-2\sqrt{385}}{2*-21}=\frac{-28-2\sqrt{385}}{-42} $$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(28)+2\sqrt{385}}{2*-21}=\frac{-28+2\sqrt{385}}{-42} $
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