You can study Chapter 9 Class 11 Series Ex 9.4 from here https://www.teachoo.com/2539/620/Ex-9.4--1---Find-sum-of-series-1-x-2---2-x-3---3-x-4/category/Ex-9.4/
To learn about Chapter 9 Class 11 , visit https://www.teachoo.com/subjects/cbse-maths/class-11th/ch9-11th-sequences-and-series/
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