In an A.P. if a = 1, a n = 20 and S n = 399, then n is 1. 19 2. 21 3. 38 4. 42

Given a = 1, a n = 20 and S n = 399. We know that a n = a + (n - 1)d ∴ a n = 1 + (n - 1) × d ⇒ 20 = 1 + (n - 1)d ⇒ (n - 1)d = 20 - 1 ⇒ (n - 1)d = 19 ⇒ d = . The formula for sum of A.P. is given by, Hence, Option 3 is the correct option.

If the first term of an A.P. is -5 and the common difference is 2, then the sum of its first 6 terms is
0
5
6
15

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In an A.P., if a = -5, l = 21 and S = 200, then n is equal to
50
40
32
25

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The sum of first five multiples of 3 is
45
55
65
75

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Mathematics

In an A.P. if a = 1, a_n = 20 and S_n = 399, then n is
19
21
38
42

AP GP

Related Questions

If the first term of an A.P. is -5 and the common difference is 2, then the sum of its first 6 terms is
0
5
6
15

In an A.P., if a = -5, l = 21 and S = 200, then n is equal to
50
40
32
25

The sum of first five multiples of 3 is
45
55
65
75

Mathematics

In an A.P. if a = 1, an = 20 and Sn = 399, then n is19213842

AP GP

Related Questions

If the first term of an A.P. is -5 and the common difference is 2, then the sum of its first 6 terms is05615

In an A.P., if a = -5, l = 21 and S = 200, then n is equal to50403225

The sum of first five multiples of 3 is45556575

In an A.P. if a = 1, a_n = 20 and S_n = 399, then n is
19
21
38
42

If the first term of an A.P. is -5 and the common difference is 2, then the sum of its first 6 terms is
0
5
6
15

In an A.P., if a = -5, l = 21 and S = 200, then n is equal to
50
40
32
25

The sum of first five multiples of 3 is
45
55
65
75