Question

A student read common difference of an A.P. as - 3 instead of 3 and obtained the sum of first 10 terms as - 30. Then the actual sum of first 10 terms is equal to

• A. 240
• B. 120
• C. 300
• D. 180

Answer 1

Answer: (A)

240

Answer 2

$S_{10} = \frac{10}{2} \left[2a+9\left(-3\right)\right] = -30$ $\Rightarrow 2a-27 = -6$ $\Rightarrow a= \frac{21}{2}$ Actual sum $S_{10} = \frac{10}{2}\left[2a+9\left(3\right)\right]$ $= 5\left[21+27\right]$ $= 5\times48 = 240$