Explicație pas cu pas:
[tex]\frac{1}{2} \times (2 + 4 + 6 + ... + 22) = \frac{1}{2} \times 2(1 + 2 + 3 + ... + 11) \\ = \frac{11 \times 12}{2} = 60 \\ [/tex]
[tex]\frac{1}{2} \times (2 + 4 + 6 + ... + 20) = \frac{1}{2} \times 2(1 + 2 + 3 + ... + 10) \\ = \frac{10 \times 11}{2} = 55 \\ [/tex]
