Đáp án `+` Giải thích các bước giải`:`

`A=3/2+3/6+...+3/420`

`1/3 A=1/2+1/6+...+1/420`

`1/3 A= 1/1.2+1/2.3+...+1/20.21`

`1/3 A= 1-1/2+1/2-1/3+...+1/20-1/21`

`1/3 A=1-1/21`

`1/3 A=20/21`

`A=20/21:1/3`

`A=20/7`

`\color{#c7c1ed}text{𓏧⛧}\color{#cdc2ed}text{⊹‧₊}\color{#d1c3ee}text{️𐙚}\color{#d6c4ee}text{𓆰ᑎ}\color{#dfc4ed}text{ᗰ}\color{#f2c5ee}text{ᗪ}\color{#fecbe9}text{𓆪⋆}\color{#feb5c8}text{˖≼}\color{#feabb9}text{𓍯𓂃}`