Question

The sum of the first 12 terms is 100. The first term is 20. Find the last term. (use Gauss method)

• A. $\frac{-20}{6}$
• B. $\frac{-10}{6}$
• C. $\frac{-15}{5}$
• D. $\frac{-30}{2}$

Answer 1

The correct option is A $\frac{-20}{6}$

Given that $S_n=100,n=12,a=20$. fast term = ?
we know that, $S_n=\frac{n}{2}$ [First term + Last term]
$100=\frac{12}{2}$ [20 + Last term]
$100=120+6\left(\text{Last term}\right)$
Last term $=\frac{-20}{6}$