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Find the sum of the first 9 terms in the following geometric series. Do not round your answer.

7+21+63+dots

Find the sum of the first 99 terms in the following geometric series. Do not round your answer.\newline7+21+63+ 7+21+63+\ldots

Full solution

Q. Find the sum of the first 99 terms in the following geometric series. Do not round your answer.\newline7+21+63+ 7+21+63+\ldots
  1. Identify first term and ratio: Identify the first term and the common ratio of the geometric series.\newlineThe first term a1a_1 is 77. To find the common ratio rr, we divide the second term by the first term.\newliner=217=3r = \frac{21}{7} = 3
  2. Use formula for sum: Use the formula for the sum of the first nn terms of a geometric series.\newlineThe formula for the sum of the first nn terms (SnS_n) of a geometric series is:\newlineSn=a1×(1rn)/(1r)S_n = a_1 \times (1 - r^n) / (1 - r), where a1a_1 is the first term, rr is the common ratio, and nn is the number of terms.
  3. Plug values for first 99 terms: Plug the values into the formula to find the sum of the first 99 terms.\newlineWe have a1=7a_1 = 7, r=3r = 3, and n=9n = 9.\newlineS9=7×(139)/(13)S_9 = 7 \times (1 - 3^9) / (1 - 3)
  4. Calculate power of 33: Calculate the power of 33 to the 99th power. 39=196833^9 = 19683
  5. Substitute value into formula: Substitute the value of 393^9 into the formula.\newlineS9=7×(119683)/(13)S_9 = 7 \times (1 - 19683) / (1 - 3)
  6. Simplify expression and denominator: Simplify the expression inside the parentheses and the denominator.\newlineS9=7×(119683)/(2)S_9 = 7 \times (1 - 19683) / (-2)\newlineS9=7×(19682)/(2)S_9 = 7 \times (-19682) / (-2)
  7. Perform multiplication and division: Perform the multiplication and division to find the sum.\newlineS9=7×9841S_9 = 7 \times 9841\newlineS9=68887S_9 = 68887

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