Question

if the sum of the first 20 th and first 19 th term of an AP are 250&225 respectively then the 20 th term of the AP is​

Answer 1

hey mate,  here is your answer thank you

Step-by-step explanation:

Let a be the first term and d be the common difference of the given AP.

Sum of the first n terms is given by

Sn = n/2 {2a + (n - 1)d}

Putting n = 10, we get

S₁₀ = 10/2 {2a + (10 - 1)d}

210 = 5 (2a + 9d)

2a + 9d = 210/5

2a + 9d = 42 (1)

Sum of the last 15 terms is 2565

⇒ Sum of the first 50 terms - sum of the first

35 terms = 2565

S₅₀ - S₃₅ = 2565

⇒ 50/2 {2a + (50 - 1)d} - 35/2 {2a + (35 - 1)d} = 2565

25 (2a + 49d) - 35/2 (2a + 34d) = 2565

⇒ 5 (2a + 49d) - 7/2 (2a + 34d) = 513

⇒ 10a + 245d - 7a + 119d = 513

⇒ 3a + 126d = 513

⇒ a + 42d = 171 .(2)

Multiply the equation (2) with 2, we get

2a + 84d = 342 (3)

Subtracting (1) from (3)

2a + 84d = 342

2a + 9d = 42

75d = 300

d= 4

Now, substituting the value of d in equation (1)

2a + 9d = 42

2a + 9*4 = 42

2a = 42 - 36

2a = 6

a = 3

So, the required AP is 3, 7, 11, 15, 19, 23, 27, 31, 35, 39

hope it was helpful