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The sum of the first 66 terms of a geometric series is 1562415624 and the common ratio is 55. What is the first term of the series?

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Q. The sum of the first 66 terms of a geometric series is 1562415624 and the common ratio is 55. What is the first term of the series?
  1. Define Terms: Let's denote the first term of the geometric series as aa, and the common ratio as rr. The sum of the first nn terms of a geometric series is given by the formula Sn=a(1rn)(1r)S_n = \frac{a(1 - r^n)}{(1 - r)}, where SnS_n is the sum of the first nn terms, aa is the first term, and rr is the common ratio. We are given that S6=15624S_6 = 15624 and r=5r = 5.
  2. Substitute Values: Substitute the given values into the sum formula for a geometric series to find the first term aa.S6=a(1r6)(1r)S_6 = \frac{a(1 - r^6)}{(1 - r)}15624=a(156)(15)15624 = \frac{a(1 - 5^6)}{(1 - 5)}
  3. Calculate Exponents: Calculate the value of 565^6 and substitute it into the equation.\newline56=156255^6 = 15625\newline15624=a(115625)/(15)15624 = a(1 - 15625) / (1 - 5)
  4. Simplify Equation: Simplify the equation by performing the operations inside the parentheses.\newline15624=a(115625)/(4)15624 = a(1 - 15625) / (-4)\newline15624=a(15624)/(4)15624 = a(-15624) / (-4)
  5. Divide and Solve: Simplify the equation further by dividing 15624-15624 by 4-4.15624=a(156244)15624 = a\left(\frac{-15624}{-4}\right)15624=a(3916)15624 = a(3916)
  6. Divide and Solve: Simplify the equation further by dividing 15624-15624 by 4-4.15624=a(15624)/(4)15624 = a(-15624) / (-4)15624=a(3916)15624 = a(3916)Solve for aa by dividing both sides of the equation by 39163916.a=15624/3916a = 15624 / 3916a=4a = 4

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