Question

Sum of first n terms of an arithmetic sequence is $5n^2+2n$ What is the 21st term of this sequence ?

• A. 210
• B. 207
• C. 209
• D. 189

Answer 1

Answer:

B. 207

$S_n=5n^2+2n$

$S_1=5 \times 1^2+2 \times 1=7(a_1)$

$S_2=5 \times 2^2+2 \times 2 = 24$

$a_1+a_2=24$

$a_2=24-7=17$

Series = 7, 17, 27, ...

a=7 ; d=10

$a_{21}=a+20d=7+20x10=207$