Question

5. Mean= 5, number of terms (n) = ?, if sum of all terms = 65.

Answer 1

Answer:

Find the sum of the first 20 terms of the arithmetic series if a1=5 and a20=62 .

S20=20(5 + 62)2S20=670

Example 2:

Find the sum of the first 40 terms of the arithmetic sequence

2,5,8,11,14,⋯

First find the 40 th term:

a40=a1+(n−1)d        =2+39(3)=119

Then find the sum:

Sn=n(a1+an)2S40=40(2 + 119)2=2420

Example 3:

Find the sum:

∑k=150(3k+2)

First find a1 and a50 :

a1=3(1)+2=5a20=3(50)+2=152

Then find the sum:

Sk=k(a1 + ak)2S50=50(5 + 152)2=3925

Answer 2

Answer:

65 terms why because

there will be

n=65