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fie a,b,c,d apartine R,astfel incat a+b=20 si c+d=30.Calculati ac+ad+bc+bd.    REZOLVARE COMPLETA!

Răspuns :

ac+ad+bc+bd=a(c+d)+b(c+d)=20a+20b=20(a+b)=20*30=600
a+b=20
c+d=30
ac+ad+bc+bd=600
=a(c+d)+b(c+d)
=(c+d)(a+b)
=30×20
=600