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Find the sum upto n terms of the series `a+aa+aaa+aaaa+"......",AA a in N " and "1 le a le 9.` |
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Answer» Let ` S = a+aa+aaa+aaaa+"......" " upto n terms " ` ` = a(1+11+111+1111+"......" "upto n terms") ` ` = (a)/(9)(9+99+999+9999+"......" "upto n terms") ` ` = (a)/(9){(10^(1)-1)+(10^(2)-1)+(10^(3)-1)+(10^(4)-1)+"......" "upto n terms"} ` ` = (a)/(9){10+10^(2)+10^(3)+"......" "upto n terms")-(1+1+1+"......" "upto n terms")}` ` = (a)/(9){(10(10^(n)-1))/(10-1)-n}=(a)/(9){(10)/(9)(10^(n)-1)-n}` |
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