Intersection of Two Arithmetic Sequences

00:00 7 marks

  • Build Confidence
  • SL & HL
  • Tier 3
  • Arithmetic sequences
  • System of equations
  • Series
  • Quadratic equations

Determine the intersection point of the partial sums of two arithmetic progressions by completing the following parts.

Part a

4 minutes 3 marks

Question

Given two arithmetic progressions (AP1 and AP2) with unknown first terms and common differences, the sum of the first \(n\) terms of AP1 is given by \(S_1 = an + \frac{n(n-1)}{2}d_1\) and the sum of the first \(n\) terms of AP2 is given by \(S_2 = bn + \frac{n(n-1)}{2}d_2\). Form an equation by equating \(S_1\) and \(S_2\) that represents the condition under which their partial sums are equal.



Hint Hint

Hint

Express both sums in terms of \(n\), then set \(S_1 = S_2\) to find the condition for equal partial sums.

  • Answer Answer

Complete Answer

The equation is \(an + \frac{n(n-1)}{2}d_1 = bn + \frac{n(n-1)}{2}d_2\).

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Part b

3 minutes 4 marks

Question

Assuming \(d_1 \neq d_2\) and given specific values \(a=2\), \(d_1=3\), \(b=5\), and \(d_2=1\), substitute these values into the equation from part (a) and solve for \(n\) such that \(S_1 = S_2\).



Hint Hint

Hint

Substitute the given values into the equation derived in part (a) and solve the resulting quadratic equation for \(n\). Consider the validity of solutions in the context of the number of terms in a series.

  • Answer Answer

Complete Answer

n = 4

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