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a^2-21a+104=0
a = 1; b = -21; c = +104;
Δ = b2-4ac
Δ = -212-4·1·104
Δ = 25
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:$a_{1}=\frac{-b-\sqrt{\Delta}}{2a}$$a_{2}=\frac{-b+\sqrt{\Delta}}{2a}$$\sqrt{\Delta}=\sqrt{25}=5$$a_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-21)-5}{2*1}=\frac{16}{2} =8 $$a_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-21)+5}{2*1}=\frac{26}{2} =13 $
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