Question

Fie a,b,c,d (apartine) R , astfel incat a+b=20 si c+d= 30. calculati ac+ad+bc+bd

Answer 1

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

Answer 2

$\displaystyle ac+ad+bc+bd= \\ \\ =a(c+d)+b(c+d)= \\ \\ =(a+b)(c+d)= \\ \\ =20 \cdot 30= \\ \\ =600.$