Progression: nth Term of a Progression
This lesson note explains how to determine the nth term in a progression. We consider both:
A. Arithmetic Progression (A.P.)
The nth term of an A.P. is given by:
an = a + (n - 1)d, where:
- a is the first term,
- d is the common difference, and
- n is the term number.
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Example 1: For a = 2 and d = 3, find the 5th term.
a5 = 2 + (5 - 1)×3 = 2 + 12 = 14.
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Example 2: For a = 7 and d = 2, find the 8th term.
a8 = 7 + (8 - 1)×2 = 7 + 14 = 21.
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Example 3: For a = 1 and d = 4, find the 6th term.
a6 = 1 + (6 - 1)×4 = 1 + 20 = 21.
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Example 4: For a = 10 and d = -2, find the 4th term.
a4 = 10 + (4 - 1)×(-2) = 10 - 6 = 4.
B. Geometric Progression (G.P.)
The nth term of a G.P. is given by:
an = a × r(n - 1), where:
- a is the first term,
- r is the common ratio, and
- n is the term number.
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Example 1: For a = 2 and r = 2, find the 5th term.
a5 = 2 × 2(5 - 1) = 2 × 16 = 32.
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Example 2: For a = 3 and r = 3, find the 4th term.
a4 = 3 × 3(4 - 1) = 3 × 27 = 81.
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Example 3: For a = 5 and r = 0.5, find the 6th term.
a6 = 5 × (0.5)(6 - 1) = 5 × 0.03125 ≈ 0.15625.
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Example 4: For a = 10 and r = 0.8, find the 3rd term.
a3 = 10 × (0.8)(3 - 1) = 10 × 0.64 = 6.4.
Progression: Sum of A.P. and G.P.
This section explains how to find the sum of the first n terms in both Arithmetic and Geometric progressions.
A. Sum of an Arithmetic Progression (A.P.)
The sum of the first n terms of an A.P. is given by:
Sn = n/2 [2a + (n - 1)d] or Sn = n/2 (first term + last term).
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Example 1: For a = 2 and d = 3 (5 terms),
S5 = 5/2 [2×2 + (5 - 1)×3] = 5/2 × 16 = 40.
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Example 2: For a = 7 and d = 2 (8 terms),
S8 = 8/2 [2×7 + (8 - 1)×2] = 4 × 28 = 112.
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Example 3: For a = 1 and d = 4 (6 terms),
S6 = 6/2 [2×1 + (6 - 1)×4] = 3 × 22 = 66.
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Example 4: For a = 10 and d = -2 (4 terms),
S4 = 4/2 [2×10 + (4 - 1)×(-2)] = 2 × 14 = 28.
B. Sum of a Geometric Progression (G.P.)
The sum of the first n terms of a G.P. (for r ≠ 1) is given by:
Sn = a (rn - 1) / (r - 1) (if r > 1) or Sn = a (1 - rn) / (1 - r) (if r < 1).
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Example 1: For a = 2 and r = 2 (5 terms),
S5 = 2 × (25 - 1)/(2 - 1) = 2 × 31 = 62.
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Example 2: For a = 3 and r = 3 (4 terms),
S4 = 3 × (34 - 1)/(3 - 1) = 3 × 40 = 120.
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Example 3: For a = 5 and r = 0.5 (6 terms),
S6 = 5 × (1 - 0.56)/(1 - 0.5) ≈ 9.84.
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Example 4: For a = 10 and r = 0.8 (3 terms),
S3 = 10 × (1 - 0.83)/(1 - 0.8) = 24.4.
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