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Consider an incomplete pyramid of balls on a square base having 18 layers; and having 13 balls on each side of the top layer. Then the total number N of balls in that pyramid satisfiesA. `9000 lt N lt 10000`B. `8000 lt N lt 9000`C. `7000 lt N lt 8000`D. `10000 lt N lt 12000` |
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Answer» Correct Answer - B Top layer has `(13 xx 13)` balls similarly are layer below top layer will have `(14 xx 14)` balls We have 18 layer So total number of balls `N=(13)^(2)+(14)^(2)+……..+(30)^(2)` `N = (30xx31xx61)/6-(12xx13xx25)/6` `N= 8805`. |
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