+) Ta có: B= 1/1.2+1/3.4+1/5.6+...+1/2019.2020
Lại có: 1/1.2>1/3.4; 1/3.4>1/5.6
⇒1/2018.2019>1/2019.2020
⇒B <1/1.2+1/2.3+1/4.5+...+1/2018.2019
⇒B< 1-1/2019<1 (1)
+) A=2^2018/2^2018+3^2019 +3^2019/3^2019+5^2020 +5^2020/5^2020+2^2018 > 2^2018/2^2019+3^2019+5^2020 +3^2019/3^2019+5^2020+ 2^2018 +5^2020/5^2020+2^2018 +3^2019 >1 (2)
Từ (1) và (2) ⇒A>B
nhớ tick cho mk nha