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CBSE Class 10 Answered

If the sum of n terms of an AP is 4n*-n, find the 12th term.
  •      Here ' * ' refers to square 
Asked by godson babuji | 06 Nov, 2014, 12:49: AM
answered-by-expert Expert Answer
G i v e n space t h a t space s u m space o f space n space t e r m s space o f space a n space A P space i s space 4 n squared minus n rightwards double arrow S subscript n equals 4 n squared minus n S i m i l a r l y comma S subscript n minus 1 end subscript equals 4 open parentheses n minus 1 close parentheses squared minus open parentheses n minus 1 close parentheses space space space space space space space space space equals 4 open square brackets n squared plus 1 minus 2 n close square brackets minus n plus 1 space space space space space space space space space equals 4 n squared plus 4 minus 8 n minus n plus 1 space space space space space space space space space equals 4 n squared minus 9 n plus 5 W e space k n o w space t h a t comma space t subscript n equals S subscript n minus S subscript n minus 1 end subscript space space space equals 4 n squared minus n minus open parentheses 4 n squared minus 9 n plus 5 close parentheses space space space equals 4 n squared minus n minus 4 n squared plus 9 n minus 5 space space space equals 8 n minus 5 T h u s comma space t subscript 12 equals 8 cross times 12 minus 5 equals 91
Answered by Vimala Ramamurthy | 06 Nov, 2014, 09:05: AM

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