Practice question · Numerical answer
An arithmetic sequence has first term 4 and common difference 7. Find the twentieth term.
Hints
- Use a_n = a_1 + (n - 1)d.
- Reaching the twentieth term takes 19 steps, not 20.
Show the answer
137
Why
a_20 = 4 + 19(7) = 4 + 133 = 137. The factor is n - 1, not n, because the first term needs no steps at all, using 20 x 7 gives 144 and is the standard off-by-one error in this formula.
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