An arithmetic series has first term aa and common difference dd, where aa and dd are non-zero. If the first, fifth and eleventh terms of the arithmetic series are equal to the first, second and third terms of a geometric series respectively, find the exact sum of the first ten terms of the geometric series, in the form kaka, where kk is a simplified rational number.

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Worked solution

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1. Set up an equation by expressing the terms using the arithmetic progression and equating the ratio of consecutive terms for the geometric progression.

Let the terms of the geometric series be T1T_1, T2T_2, and T3T_3. From the arithmetic progression, we have:
T1=aT_1 = a
T2=a+4dT_2 = a + 4d
T3=a+10dT_3 = a + 10d

Since these terms form a geometric series, the ratio between consecutive terms must be equal:
a+4da=a+10da+4d\frac{a + 4d}{a} = \frac{a + 10d}{a + 4d}

2. Solve the equation to find a relationship between $d$ and $a$.

Cross-multiplying to solve for dd:
(a+4d)2=a(a+10d)(a + 4d)^2 = a(a + 10d)
a2+8ad+16d2=a2+10ada^2 + 8ad + 16d^2 = a^2 + 10ad
16d2−2ad=016d^2 - 2ad = 0
2d(8d−a)=02d(8d - a) = 0

Since it is given that d≠0d \neq 0, we can divide by 2d2d (or select the non-zero root):
8d−a=08d - a = 0
d=a8d = \frac{a}{8}

3. Calculate the common ratio $r$ of the geometric series.

Substitute d=a8d = \frac{a}{8} back into the ratio expression:
r=a+4(a8)ar = \frac{a + 4\left(\frac{a}{8}\right)}{a}
r=a+12aar = \frac{a + \frac{1}{2}a}{a}
r=32r = \frac{3}{2}

4. Use the formula for the sum of a finite geometric series to find the sum of the first ten terms.

The sum of the first 10 terms is given by S10=a(r10−1)r−1S_{10} = \frac{a(r^{10} - 1)}{r - 1}:
S10=a((32)10−1)32−1S_{10} = \frac{a\left(\left(\frac{3}{2}\right)^{10} - 1\right)}{\frac{3}{2} - 1}
S10=a(590491024−1)12S_{10} = \frac{a\left(\frac{59049}{1024} - 1\right)}{\frac{1}{2}}
S10=2a(59049−10241024)S_{10} = 2a\left(\frac{59049 - 1024}{1024}\right)
S10=2a(580251024)S_{10} = 2a\left(\frac{58025}{1024}\right)
S10=58025512aS_{10} = \frac{58025}{512}a

Answer

58025512a\frac{58025}{512}a

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