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X^2-400X+8400=0
a = 1; b = -400; c = +8400;
Δ = b2-4ac
Δ = -4002-4·1·8400
Δ = 126400
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}$$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}$
The end solution:
$\sqrt{\Delta}=\sqrt{126400}=\sqrt{1600*79}=\sqrt{1600}*\sqrt{79}=40\sqrt{79}$$X_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-400)-40\sqrt{79}}{2*1}=\frac{400-40\sqrt{79}}{2} $$X_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-400)+40\sqrt{79}}{2*1}=\frac{400+40\sqrt{79}}{2} $
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