How do I find the sum of first n terms of an AP if I know the first and last terms?
I have an arithmetic progression where first term a = 3, last term l = 51, and I need to find the sum. I know there are two formulas for sum of AP and I do not know which to use.
1 Answer
VNVikram Nair
✓ Accepted
· 15d ago
▲ 10
When you know first term (a) and last term (l), use: Sn = n/2 * (a + l). This is simpler. When you know a and common difference d but not last term, use: Sn = n/2 * (2a + (n-1)d). For your example, you still need n. Find d and use the nth term formula: l = a + (n-1)d. With a=3, l=51 and d=? If d=4: 51 = 3 + (n-1)*4, so 48 = (n-1)*4, n-1=12, n=13. Sum = 13/2 * (3+51) = 13/2 * 54 = 351.
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