Răspuns:
numărul care lipsește este 11
Explicatie:
A(1)=1=1
A(1)=1=1A(2)=A(1)+1=2
A(1)=1=1A(2)=A(1)+1=2A(3)=A(2)+2=A(1)+3=4
A(1)=1=1A(2)=A(1)+1=2A(3)=A(2)+2=A(1)+3=4A(4)=A(3)+4-1=A(1)+3+3=7
A(1)=1=1A(2)=A(1)+1=2A(3)=A(2)+2=A(1)+3=4A(4)=A(3)+4-1=A(1)+3+3=7A(5)=A(4)+5-1=A(1)+6+4=11