CBSE Class 10 Answered
find the sum of first 15 term of an A.P whose nth term is 3-2n?
Asked by himanshi.updel | 13 Oct, 2018, 11:23: AM
![begin mathsize 18px style straight S subscript straight n equals straight n over 2 open parentheses 2 straight a plus open parentheses straight n minus 1 close parentheses straight d close parentheses
straight S subscript 15 equals 15 over 2 open parentheses 2 straight a plus open parentheses 15 minus 1 close parentheses straight d close parentheses
equals 15 open parentheses straight a plus open parentheses 7 close parentheses straight d close parentheses
straight a subscript straight n equals 3 minus 2 straight n
straight a plus nd minus straight d equals 3 minus 2 straight n
straight a minus straight d plus nd equals 3 minus 2 straight n
straight a minus straight d space equals space 3 space space and space nd space equals negative 2 straight n....... straight d equals negative 2
so comma space straight a space equals 1...... substitute end style](https://images.topperlearning.com/topper/tinymce/cache/ccac5b094e877143a6f8161cf642cf67.png)
Answered by Arun | 13 Oct, 2018, 17:55: PM
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