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If ab + xy - xb = 0 and bc + yz - cy = 0, then what \(\frac{x}{a} + \frac{c}{z} \)  equal to?

This question was previously asked in
CDS I 2020 Elementary Mathematics Previous Year Paper (02-Feb-2020)
The correct answer is

1

Understanding the Problem

We are given two algebraic equations involving variables a, b, c, x, y, and z. Our goal is to find the value of the expression \( \frac{x}{a} + \frac{c}{z} \) using these equations.

The given equations are:

  1. \( ab + xy - xb = 0 \)
  2. \( bc + yz - cy = 0 \)

We need to manipulate these equations to find expressions for \( \frac{x}{a} \) and \( \frac{c}{z} \) and then add them together.

Solving the First Equation for \( \frac{x}{a} \)

Let's take the first equation:

\( ab + xy - xb = 0 \)

We want to isolate terms involving x and a to eventually get \( \frac{x}{a} \). Let's rearrange the terms to group those with x:

\( xy - xb = -ab \)

Factor out x from the left side:

\( x(y - b) = -ab \)

To get \( \frac{x}{a} \), we can divide both sides by \( a(y - b) \). We assume \( a \neq 0 \) and \( y \neq b \) to avoid division by zero.

\( \frac{x(y - b)}{a(y - b)} = \frac{-ab}{a(y - b)} \)

Simplify both sides:

\( \frac{x}{a} = \frac{-b}{y - b} \)

We can rewrite the right side by multiplying the numerator and denominator by -1:

\( \frac{x}{a} = \frac{-b}{-(b - y)} = \frac{b}{b - y} \)

So, we have an expression for \( \frac{x}{a} \):

\( \frac{x}{a} = \frac{b}{b - y} \quad \text{(Equation 3)} \)

Solving the Second Equation for \( \frac{c}{z} \)

Now, let's take the second equation:

\( bc + yz - cy = 0 \)

We want to isolate terms involving c and z to eventually get \( \frac{c}{z} \). Let's rearrange the terms to group those with c:

\( bc - cy = -yz \)

Factor out c from the left side:

\( c(b - y) = -yz \)

To get \( \frac{c}{z} \), we can divide both sides by \( z(b - y) \). We assume \( z \neq 0 \) and \( b \neq y \) to avoid division by zero.

\( \frac{c(b - y)}{z(b - y)} = \frac{-yz}{z(b - y)} \)

Simplify both sides:

\( \frac{c}{z} = \frac{-y}{b - y} \)

We can rewrite the right side:

\( \frac{c}{z} = \frac{-y}{b - y} = \frac{y}{-(b - y)} = \frac{y}{y - b} \)

So, we have an expression for \( \frac{c}{z} \):

\( \frac{c}{z} = \frac{y}{y - b} \quad \text{(Equation 4)} \)

Adding the Expressions \( \frac{x}{a} \) and \( \frac{c}{z} \)

Now we need to find the sum \( \frac{x}{a} + \frac{c}{z} \). We use the expressions from Equation 3 and Equation 4.

\( \frac{x}{a} + \frac{c}{z} = \frac{b}{b - y} + \frac{y}{y - b} \)

Notice that the denominators are related: \( y - b = -(b - y) \). We can rewrite the second term to have the same denominator as the first term:

\( \frac{y}{y - b} = \frac{y}{-(b - y)} = -\frac{y}{b - y} \)

Substitute this back into the sum:

\( \frac{x}{a} + \frac{c}{z} = \frac{b}{b - y} + \left(-\frac{y}{b - y}\right) \)

\( \frac{x}{a} + \frac{c}{z} = \frac{b}{b - y} - \frac{y}{b - y} \)

Now that the denominators are the same, we can combine the numerators:

\( \frac{x}{a} + \frac{c}{z} = \frac{b - y}{b - y} \)

Assuming \( b - y \neq 0 \), the expression \( \frac{b - y}{b - y} \) simplifies to 1.

\( \frac{x}{a} + \frac{c}{z} = 1 \)

Conclusion

Based on the given equations and assuming the necessary variables are non-zero (specifically \( a \neq 0, z \neq 0, b \neq y \)), the value of \( \frac{x}{a} + \frac{c}{z} \) is 1.

Step Action Result
1 Rearrange first equation: \(ab + xy - xb = 0\) \(x(y - b) = -ab\)
2 Solve for \( \frac{x}{a} \) \( \frac{x}{a} = \frac{b}{b - y} \)
3 Rearrange second equation: \(bc + yz - cy = 0\) \(c(b - y) = -yz\)
4 Solve for \( \frac{c}{z} \) \( \frac{c}{z} = \frac{y}{y - b} \)
5 Add \( \frac{x}{a} \) and \( \frac{c}{z} \) \( \frac{x}{a} + \frac{c}{z} = \frac{b}{b - y} + \frac{y}{y - b} \)
6 Simplify the sum \( \frac{b - y}{b - y} = 1 \)

Revision Table: Key Steps in Solving

This table summarizes the main steps taken to solve the problem involving the algebraic equations and find the value of the expression.

Concept Used Application
Rearranging Equations Grouping terms with specific variables (x, c).
Factoring Pulling out common variables (x, c).
Division Dividing both sides to isolate desired ratios \( \frac{x}{a} \) and \( \frac{c}{z} \).
Combining Fractions Adding fractions with related denominators.
Simplification Reducing the final expression to its simplest value.

Additional Information: Algebraic Manipulation Tips

Solving problems like this often involves basic algebraic techniques. Here are a few tips:

  • Isolating Variables: To solve for a specific variable or expression, use inverse operations (addition/subtraction, multiplication/division) to move other terms to the opposite side of the equation.
  • Factoring: Look for common factors in terms to simplify expressions, as seen when factoring out x and c in the given equations.
  • Combining Fractions: To add or subtract fractions, they must have a common denominator. You can rewrite fractions by multiplying the numerator and denominator by the same non-zero value. Remember that \( a - b = -(b - a) \).
  • Checking for Restrictions: When dividing, always consider what values would make the denominator zero. In this problem, we assumed \( a \neq 0, z \neq 0 \), and \( b - y \neq 0 \) (i.e., \( b \neq y \)). If \( b = y \), the original equations simplify differently, possibly leading to conditions like \( ab = 0 \) and \( bz = 0 \), which might make the expression \( \frac{x}{a} + \frac{c}{z} \) undefined or require a different approach depending on the values of a and z. However, standard problems like this usually imply the generic case where denominators are non-zero.
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