The sum of the first 100 terms of an arithmetic progression is 10,000. The first, second and fifth terms of this progression are three consecutive terms of a geometric progression. Find the first term and the non-zero common difference of the arithmetic progression.

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Worked solution

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1. Form an equation using the sum of the first 100 terms of the arithmetic progression.

Let the first term be aa and the common difference be dd.
Using the sum formula Sn=n2[2a+(n−1)d]S_n = \frac{n}{2}[2a + (n-1)d]:
S100=1002[2a+99d]=10000S_{100} = \frac{100}{2}[2a + 99d] = 10000
50(2a+99d)=1000050(2a + 99d) = 10000
2a+99d=200— (1)2a + 99d = 200 \quad \text{--- (1)}

2. Identify the $1^\text{st}$, $2^\text{nd}$ and $5^\text{th}$ terms of the AP.

The respective terms in the AP are:
T1=aT_1 = a
T2=a+dT_2 = a + d
T5=a+4dT_5 = a + 4d

3. Form an equation using the property of the consecutive terms of a geometric progression.

Since T1T_1, T2T_2 and T5T_5 are three consecutive terms of a GP, they share a common ratio:
T2T1=T5T2\frac{T_2}{T_1} = \frac{T_5}{T_2}
a+da=a+4da+d\frac{a+d}{a} = \frac{a+4d}{a+d}

4. Simplify the GP equation to find the relationship between $a$ and $d$.

Cross-multiplying gives:
(a+d)2=a(a+4d)(a+d)^2 = a(a+4d)
a2+2ad+d2=a2+4ada^2 + 2ad + d^2 = a^2 + 4ad
d2−2ad=0d^2 - 2ad = 0
d(d−2a)=0d(d - 2a) = 0
Since the question states the common difference is non-zero (d≠0d \neq 0):
d=2a— (2)d = 2a \quad \text{--- (2)}

5. Solve the simultaneous equations to find $a$ and $d$.

Substitute equation (2) into equation (1):
2a+99(2a)=2002a + 99(2a) = 200
2a+198a=2002a + 198a = 200
200a=200  ⟹  a=1200a = 200 \implies a = 1
Substitute a=1a = 1 back into equation (2):
d=2(1)=2d = 2(1) = 2

Answer

a=1a = 1, d=2d = 2

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