Question

5th term and 12th term are 30 and 65 .what is the sum of 20 term

pls do step by step​

Answer 1

In general,

Nth term of A.P = a + (n-1)d,

5th term of A.P = a + 4d = 30 ........(1),

12h term of A.P = a + 11d = 65......(2),

Solving (1) and (2)

7d = 35,

=> d = 5,

=> a + 4(5) = 30,

=> a + 20 = 30,

=> a = 10,

Sum of n terms of A.P = (n/2) *(2a+(n-1)d),

Sum of first 20 terms = 10*(20+19*5)

=> 10(115)

=> 1150,

Therefore the sum of first 20 numbers of A.P is 1150,

Hope you understand ,

Have a great day !!

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Answer 2

Answer:

The sum of first 20 terms is 1150. Step-by-step explanation: Given : a5 = 30, a12 = 65 ,. Case 1 : By using the formula ,an = a + (n - 1)d