Question

Sum of first n terms of an arithmetic sequence is 5n² + 2n. What is the 21st term of the sequence ?

• A. 102
• B. 107
• C. 202
• D. 207

Answer 1

Answer:

D. 207

$Pn^2+qn$

$p=\frac{d}{2}$

$p+q=f$

$d=5\times 2 = 10$

$5+2=f$

$x_{21}=f+20d=7+20\times10=207$