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Find the sum of first 21 terms of AP whose 2nd term is 8 and 4th term is 14 |
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Answer» -----(1)a+3d=14-----(2)(2)÷(1)a+3d=14a+1d=8-. -. -------------- 2d=6 d=3sub d in EQN (1) or (2) a+1(3)=8 a+3=8 a=8-3 a=5s21=n/2(2a+(n-1)d) =21/2(2(5)+(21-1)3) =10.5(10+20×3) =10.5(70) =735hope it's HELPFUL..MARK me as BRILLIANT |
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