x²-mx+5=0 ∆=m²-4·5=m²-20 x1,2=(m±√(m²-20) )/2 1/x1+1/x2=3 1/(m+√(m²-20))/2+1/(m-√(m²-20)/2=3 2/(m+√(m²-20))+2/m-√(m²-20))=3 2(m-√(m²-20))/(m+√(m²-20))(m-√(m²-20))+2(m+√(m²-20))/(m+√(m²-20))(m-√(m²-20))=3 (2m-2√(m²-20)+2m+2√(m²-20)) / (m²-m²-20)=3 4m/-20=3 m=3·(-20):4=-15