Which term of the AP : 21, 18, 15,... is `- 81`? Also, is any term 0? Give reason for your answer.

Explanation

To solve the problem, we need to find which term of the arithmetic progression (AP) 21, 18, 15,... is equal to -81, and whether there is any term in this AP that equals 0.

### Step-by-step Solution:

1. **Identify the first term (a) and the common difference (d)**:
- The first term \( a = 21 \)
- The common difference \( d = 18 - 21 = -3 \)

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