Solve Simultaneous Equations: 4x - 7/y = -3 and 5x + 2/y = 7

Simultaneous Equations with Reciprocal Terms

4x7y=3 4x-\frac{7}{y}=-3

5x+2y=7 5x+\frac{2}{y}=7

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Solve
00:05 Let's multiply each equation so we can subtract between them
00:24 Now let's subtract between the equations
00:27 Let's simplify what we can
00:39 Let's collect terms
00:49 Let's isolate Y
00:58 This is the value of Y
01:04 Now let's substitute Y to find the value of X
01:14 Let's isolate X
01:25 And this is the solution to the question

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

4x7y=3 4x-\frac{7}{y}=-3

5x+2y=7 5x+\frac{2}{y}=7

2

Step-by-step solution

To solve the system of equations, we will apply the substitution method:

We start with the given equations:

4x7y=3 4x - \frac{7}{y} = -3 (Equation 1)

5x+2y=7 5x + \frac{2}{y} = 7 (Equation 2)

Let's start by solving Equation 1 for 1y \frac{1}{y} :

4x7y=3 4x - \frac{7}{y} = -3

Rearrange to isolate 1y \frac{1}{y} :

7y=34x -\frac{7}{y} = -3 - 4x

Multiply through by -1 to simplify:

7y=4x+3 \frac{7}{y} = 4x + 3

1y=4x+37 \frac{1}{y} = \frac{4x + 3}{7} (Equation 3)

Now, substitute Equation 3 into Equation 2:

5x+2y=7 5x + \frac{2}{y} = 7

Replace 1y \frac{1}{y} from Equation 3:

5x+2(4x+37)=7 5x + 2 \left(\frac{4x + 3}{7}\right) = 7

Multiply both sides of the equation by 7 to eliminate the fraction:

35x+2(4x+3)=49 35x + 2(4x + 3) = 49

Expand and simplify:

35x+8x+6=49 35x + 8x + 6 = 49

43x+6=49 43x + 6 = 49

Subtract 6 from both sides:

43x=43 43x = 43

Divide by 43:

x=1 x = 1

Substitute x=1 x = 1 back into Equation 3 to solve for y y :

1y=4(1)+37 \frac{1}{y} = \frac{4(1) + 3}{7}

1y=4+37 \frac{1}{y} = \frac{4 + 3}{7}

1y=77 \frac{1}{y} = \frac{7}{7}

1y=1 \frac{1}{y} = 1

Therefore, y=1 y = 1

The solution to the system of equations is x=1,y=1 x = 1, y = 1 .

3

Final Answer

x=1,y=1 x=1,y=1

Key Points to Remember

Essential concepts to master this topic
  • Substitution Method: Solve one equation for a variable, then substitute
  • Technique: Let u=1y u = \frac{1}{y} to convert 7y \frac{7}{y} to 7u 7u
  • Check: Substitute x=1,y=1 x=1, y=1 : 4(1)71=3 4(1) - \frac{7}{1} = -3

Common Mistakes

Avoid these frequent errors
  • Treating reciprocal terms like regular fractions
    Don't solve 7y \frac{7}{y} by cross-multiplying like a proportion = wrong algebraic manipulation! Reciprocal terms need isolation first. Always rearrange to get 1y \frac{1}{y} by itself, then substitute.

Practice Quiz

Test your knowledge with interactive questions

\( \begin{cases} x+y=8 \\ x-y=6 \end{cases} \)

FAQ

Everything you need to know about this question

Why can't I just cross-multiply the fractions?

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These aren't proportions! We have addition and subtraction with reciprocal terms. Cross-multiplication only works when you have one fraction equals another fraction, not mixed operations.

What's the best way to handle the reciprocal terms?

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Use substitution! Let u=1y u = \frac{1}{y} , so your equations become 4x7u=3 4x - 7u = -3 and 5x+2u=7 5x + 2u = 7 . Much easier to solve!

How do I know which variable to solve for first?

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Look for the easier coefficient! In this problem, solving the first equation for 1y \frac{1}{y} gives us a clean expression to substitute into the second equation.

What if I get y = 0 as an answer?

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Check your work immediately! If y=0 y = 0 , then 1y \frac{1}{y} is undefined. This means you made an error somewhere - go back and review your algebra.

Can I use elimination instead of substitution?

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Yes! Multiply the first equation by 2 and the second by 7, then add to eliminate the 1y \frac{1}{y} terms. Both methods work, but substitution is often clearer with reciprocal terms.

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