>120
thì tính số số từ 1 đến 119 có bao nhiu số thì lấy: 119 -1+1 =119
rồi lấy 118 chia 2=59 cặp (tính thêm 60 nữa là 119)
rồi lấy tổng (119+1) + (118+2) +....+(59+61) +60
mỗi cặp đều có tổng là 120 thì lấy: 120.59+60=7140
OK
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