Question

Find the first term. The sum of the first 100 terms is 1,200. The last term is 150. (use Gauss method)

• A. 126
• B. -126
• C. 125
• D. -125

Answer 1

The correct option is B -126

Using Gauss method,
$S_n=\frac{n}{2}$ [First term + Last term]
$1,200=\frac{100}{2}[a_1+150]$
$1,200=50a_1+7,500$
$1,200-7,500=50a_1$
$a_1=-126$