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If \(\frac{p+q}{q+r}=\frac{r+s}{s+p}\); \((q+r) \neq 0\), \((s+p) \neq 0\), then which one of the following is correct?

This question was previously asked in
CDS 2 2025 Maths Question Paper (14-Sep-2025)
The correct answer is
Either \(p+q+r+s=0\) or \(p=r\)

Solving the Algebraic Equation \(\frac{p+q}{q+r}=\frac{r+s}{s+p}\)

This problem requires us to find a relationship between the variables \(p\), \(q\), \(r\), and \(s\) given the equation \(\frac{p+q}{q+r}=\frac{r+s}{s+p}\). We are also given that the denominators \((q+r)\) and \((s+p)\) are not equal to zero, ensuring the equation is valid.

Step-by-Step Derivation of Variable Relationships

We start with the given equation:

\(\frac{p+q}{q+r}=\frac{r+s}{s+p}\)

To simplify this equation, we can cross-multiply:

\((p+q)(s+p) = (r+s)(q+r)\)

Next, we expand both sides of the equation:

Left side expansion: \(ps + p \cdot p + qs + qp = ps + p^2 + qs + qp\)

Right side expansion: \(rq + r \cdot r + sq + sr = rq + r^2 + sq + sr\)

Now, we set the expanded expressions equal:

\(ps + p^2 + qs + qp = rq + r^2 + sq + sr\)

To find the relationship, we rearrange the terms by moving all terms to one side:

\(p^2 + ps + qp + qs - rq - r^2 - sq - sr = 0\)

Let's group the terms strategically to simplify the expression:

\((p^2 - r^2) + (ps - sr) + (qp - rq) + (qs - sq) = 0\)

Now, we factor each group:

  • \(p^2 - r^2\) factors as \((p-r)(p+r)\).
  • \(ps - sr\) factors as \(s(p-r)\).
  • \(qp - rq\) factors as \(q(p-r)\).
  • \(qs - sq\) simplifies to \(0\).

Substituting these factored forms back into the equation:

\((p-r)(p+r) + s(p-r) + q(p-r) + 0 = 0\)

We can see that \((p-r)\) is a common factor in the first three terms. Factoring out \((p-r)\):

\((p-r) [ (p+r) + s + q ] = 0\)

Simplifying the expression inside the brackets gives:

\((p-r)(p+q+r+s) = 0\)

Conclusions from the Derived Equation

The equation \((p-r)(p+q+r+s) = 0\) holds true if and only if at least one of the factors is equal to zero. This leads to two possible scenarios:

  1. The first factor is zero: \(p-r = 0\), which means \(p = r\).
  2. The second factor is zero: \(p+q+r+s = 0\).

Therefore, the relationship derived from the original equation is that either \(p = r\) or \(p+q+r+s = 0\).

Analysis of Provided Options

Let's evaluate the given options based on our findings:

  • Option 1: \(p+q+r+s=0\). This is one of the two possible outcomes we derived.
  • Option 2: \(p=r\). This is the second possible outcome we derived.
  • Option 3: Either \(p+q+r+s=0\) or \(p=r\). This option correctly states both possible conclusions derived from the initial algebraic manipulation.
  • Option 4: None of the above. Since Option 3 is mathematically sound based on our derivation, this option is incorrect.

The correct conclusion is that one of the two conditions, \(p=r\) or \(p+q+r+s=0\), must be true.

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