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CBSE Class 10 Answered

If the sum of first 13 terms of an A.P.,  is 21 and the sum of first 21 terms is 13 . Show that sum of first 34 terms is -34. 
Asked by sucharitasahoo1 | 18 Feb, 2018, 07:38: PM
answered-by-expert Expert Answer
begin mathsize 16px style straight S subscript 13 space equals 13 over 2 open square brackets 2 straight a plus open parentheses 13 minus 1 close parentheses straight d close square brackets equals 21 13 over 2 open parentheses 2 straight a plus 12 straight d close parentheses equals 21 13 straight a plus 78 straight d equals 21......... left parenthesis straight i right parenthesis straight S subscript 21 space equals 21 over 2 open square brackets 2 straight a plus open parentheses 21 minus 1 close parentheses straight d close square brackets equals 13 21 over 2 open square brackets 2 straight a plus 20 straight d close square brackets equals 13 21 straight a plus 210 straight d equals 13........ left parenthesis ii right parenthesis Solving space left parenthesis straight i right parenthesis space and space left parenthesis ii right parenthesis space we space get space straight a equals 283 over 91 comma space straight d equals fraction numerator negative 68 over denominator 273 end fraction straight S subscript 34 equals 34 over 2 open square brackets 2 straight a plus open parentheses 34 minus 1 close parentheses straight d close square brackets straight S subscript 34 equals 17 open square brackets 2 straight a plus 33 straight d close square brackets straight S subscript 34 equals 34 straight a plus 561 straight d Put space values space of space straight a space and space straight d space in space the space above space equation space you space get space straight S subscript 34 equals negative 34 end style
Answered by Sneha shidid | 19 Feb, 2018, 11:25: AM

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