50 - 49 + 48 - 47 + 46 - 45 + ... + 4 - 3 + 2 - 1 (có 50 số)
= (50 - 49) + (48 - 47) + (46 - 45) + ... + (4 - 3) + (2 - 1) (có 25 nhóm)
= 1 + 1 + 1 + ... + 1 + 1
= 1 x 25
= 25
50-49+48-47+46-45+...+4-3+2-1
= (50-49)+(48-47)+(46-45)+...+ (4-3)+(2-1) (có 25 cặp số)
=1+1+1+1+1...+1
=1.25
=25
50-49+48-47+46-45+...+4-3+2-1
= (50-49)+(48-47)+(46-45)+...+(4-3)+(2-1) (có 25 cặp số)
=1+1+1+1+1...+1
=1.25
=25