alleryIf sum of an AP is 100 and a = 5, l = 15, number of terms is
A8
B10
C12
D15
Answer & Solution
Correct answer: B. 10
1. Use S_n = n/2 * (a + l).
2. 100 = n/2 * (5 + 15) = n/2 * 20 = 10 n.
3. So n = 10.
4. Hence there are 10 terms.
_Source: Selina Concise Mathematics Class 10, Ch on Arithmetic Progression (Mistral OCR'd PDF), section on sum formula_
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