In an arithmetic progression (A.P.), the \(n\)-th term is given by the formula:
\[ a_n = a + (n - 1)d \]
Where:
We are given:
Substitute the known values into the formula for the 10th term:
\[ a_{10} = a + (10 - 1)d \implies -19 = 8 + 9d \]
Now, solve for \(d\):
\[ -19 - 8 = 9d \implies -27 = 9d \implies d = -3 \]
Thus, the correct answer is:
\(d)\ -3\)
Assertion (A): The sum of the first fifteen terms of the AP $ 21, 18, 15, 12, \dots $ is zero.
Reason (R): The sum of the first $ n $ terms of an AP with first term $ a $ and common difference $ d $ is given by: $ S_n = \frac{n}{2} \left[ a + (n - 1) d \right]. $
| Case No. | Lens | Focal Length | Object Distance |
|---|---|---|---|
| 1 | \(A\) | 50 cm | 25 cm |
| 2 | B | 20 cm | 60 cm |
| 3 | C | 15 cm | 30 cm |