Δ = a² -4·5 = a² -20 < 0
negativ este conditia ca ecuatia nu are solutii reale
a -∞ -2√5 2√5 +∞
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a² -20 <0 + + 0 - - - - 0 + + + +
daca a ∈( -2 √5 , 2√5 )