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CBSE Class 10 Answered

if the sum of first n terms of an A.P is 4n2 - n, find the 12th term ?
Asked by nisha_vini29 | 21 Sep, 2015, 02:39: PM
answered-by-expert Expert Answer

begin mathsize 16px style For space an space straight A. straight P comma space 4 straight n squared space minus space straight n space For space straight n space equals space 12 4 straight n squared space minus space straight n space equals space open parentheses 4 cross times 12 squared close parentheses minus 12 equals open parentheses 4 cross times 144 close parentheses minus 12 space equals space 564 therefore straight S subscript 12 equals 564 For space straight n space equals space 11 4 straight n squared space minus space straight n space equals space open parentheses 4 cross times 11 squared close parentheses minus 11 equals open parentheses 4 cross times 121 close parentheses minus 11 space equals space 473 therefore straight S subscript 11 equals 473 straight t subscript 12 equals straight S subscript 12 minus straight S subscript 11 equals 564 minus 473 equals 91 Hence space the space 12 to the power of th space end exponent term space of space an space straight A. straight P space space 4 straight n squared space minus space straight n space is space 91 end style

Answered by | 21 Sep, 2015, 06:44: PM

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