Question

Q. The first and the last terms of an AP are 10 and 361 respectively. If its common difference is 9 then find the number of terms and their total sum?​

Answer 1

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Sol.

Given, first term, a = 10

Last term, al = 361

And, common difference, d = 9

Now , al =a + (n −1)d

⟹ 361 = 10 + (n − 1)9

⟹ 361 = 10 + 9n − 9

⟹ 361 = 9n + 1

⟹ 9n = 360

⟹ n = 40

Therefore, total number of terms in AP = 40

Now, sum of total number of terms of an AP is given as:

Sn = n/2 [2a + (n − 1)d]

⟹ S40 = 40/2 [2 × 10 + (40 − 1)9]

= 20[20 + 39 x 9]

=20[20 + 351]

=20 × 371 = 7420

Thus, sum of all 40 terms of AP = 7420

Answer 2

Answer:

Step-by-step explanation:

a = 10

an = 361

d = 9

an = a+(n-1)d

361 = 10+(n-1)9

361-10 = (n-1)9

351/9 = n-1

39+1 = n

n = 40

Sn = n/2(a+l)

= 40/2(10+361)

= 20(371)

= 7420