Ta có:
` D= 1+1/2 . (1+2)+1/3 . (1+2+3)+...+1/2020 . (1+2+3+...+2020) `
` <=> D= 1+ 1/2 . 3 +...+1/2020 . 2041210 `
` <=> D =1+1,5+2+...+ 1010,5 `
` <=> D = \frac{ (1010,5 + 1).[(1010,5 - 1) : 0,5 + 1] }{2} `
` <=> D = \frac{2043230}{2} `
` <=> D = 1021615 `