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
1
1. Form an equation using the sum of the first 100 terms of the arithmetic progression.
Let the first term be and the common difference be .
Using the sum formula :
2. Identify the $1^\text{st}$, $2^\text{nd}$ and $5^\text{th}$ terms of the AP.
The respective terms in the AP are:
3. Form an equation using the property of the consecutive terms of a geometric progression.
Since , and are three consecutive terms of a GP, they share a common ratio:
4. Simplify the GP equation to find the relationship between $a$ and $d$.
Cross-multiplying gives:
Since the question states the common difference is non-zero ():
5. Solve the simultaneous equations to find $a$ and $d$.
Substitute equation (2) into equation (1):
Substitute back into equation (2):
,
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