Tìm tổng các số nguyên x, biết –10 < x < 10: A. 15; B. –20;

Đáp án đúng là: C

–10 < x < 10 nên x {–9; –8; –7; –6; –5; –4; –3; –2; –1; 0; 1; 2; 3; 4; 5; 6; 7; 8; 9}

Suy ra

S = (–9) + (–8) + (–7) + (–6) + (–5) + (–4) + (–3) + (–2) + (–1) + 0 + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9

= (–9 + 9) + (–8 + 8) + … + (–1 + 1) + 0

= 0 + 0 + … + 0 + 0

= 0