If the sum of p terms of an AP is q and the sum of q terms is p, then show that the sum of `p+q` terms is `- (p+q)` , Also find the sum of first `p-q` terms (where , `pgtq`).

Explanation

To solve the problem step by step, we will use the properties of arithmetic progressions (AP) and the formulas for the sum of the first n terms of an AP.

### Step 1: Write the formula for the sum of the first n terms of an AP.
The sum of the first n terms (S_n) of an arithmetic progression is given by the formula:
\[ S_n = \frac{n}{2} \left(2a + (n - 1)d\right) \]
where \( a \) is the first term and \( d \) is the common difference.

### Step 2: Set up the equations based on the given information.
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