Solve the Fraction Equation: Balance −8 + X/3 = X + 4/9

Rational Equations with Cross-Multiplication

Solve for X:

8+x3=x+49 \frac{-8+x}{3}=\frac{x+4}{9}

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Solve
00:03 We want to isolate the unknown X
00:07 Multiply by both denominators to eliminate fractions
00:19 Simplify as much as possible
00:41 Open parentheses properly, multiply by each factor
00:48 Arrange the equation so that one side has only the unknown X
01:08 Isolate the unknown X
01:16 And this is the solution to the problem

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

Solve for X:

8+x3=x+49 \frac{-8+x}{3}=\frac{x+4}{9}

2

Step-by-step solution

To solve for x x in the equation 8+x3=x+49\frac{-8+x}{3}=\frac{x+4}{9}, we'll follow these steps:

  • Step 1: Eliminate the fractions by finding a common denominator.
  • Step 2: Simplify the resulting equation.
  • Step 3: Isolate x x to solve the equation.

Let's proceed step by step:

Step 1: The equation 8+x3=x+49\frac{-8+x}{3}=\frac{x+4}{9} contains denominators 3 and 9. The least common denominator (LCD) is 9. To eliminate the fractions, multiply every term of the equation by 9.

9×8+x3=9×x+499 \times \frac{-8+x}{3} = 9 \times \frac{x+4}{9}

Simplifying, we have:

3(8+x)=x+43(-8 + x) = x + 4

Step 2: Distribute the 3 on the left side:

3×8+3×x=x+43 \times -8 + 3 \times x = x + 4

This simplifies to:

24+3x=x+4-24 + 3x = x + 4

Step 3: Isolate x x . First, subtract x x from both sides of the equation:

24+3xx=x+4x-24 + 3x - x = x + 4 - x

This simplifies to:

24+2x=4-24 + 2x = 4

Next, add 24 to both sides to further isolate x x :

24+24+2x=4+24-24 + 24 + 2x = 4 + 24

This simplifies to:

2x=282x = 28

Finally, divide both sides by 2 to solve for x x :

x=282x = \frac{28}{2}

Simplifying this gives:

x=14x = 14

Therefore, the solution to the equation is x=14 x = 14 .

3

Final Answer

14 14

Key Points to Remember

Essential concepts to master this topic
  • LCD Method: Multiply every term by 9 to clear fractions
  • Distribution: 3(8+x)=24+3x 3(-8 + x) = -24 + 3x expands correctly
  • Verification: Substitute x = 14: 63=189 \frac{6}{3} = \frac{18}{9} gives 2 = 2 ✓

Common Mistakes

Avoid these frequent errors
  • Forgetting to multiply all terms by LCD
    Don't multiply only the numerator by 9 and ignore other terms = wrong equation! This creates an imbalanced equation where one side is cleared but the other isn't. Always multiply every single term on both sides by the LCD.

Practice Quiz

Test your knowledge with interactive questions

Solve for X:

\( 3x=18 \)

FAQ

Everything you need to know about this question

Why do we use 9 as the LCD when we have 3 and 9?

+

The LCD of 3 and 9 is 9 because 9 is divisible by both numbers. When we multiply by 9, it clears both fractions: 9×13=3 9 \times \frac{1}{3} = 3 and 9×19=1 9 \times \frac{1}{9} = 1 .

What happens if I cross-multiply instead?

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Cross-multiplication works here too! Since we have 8+x3=x+49 \frac{-8+x}{3} = \frac{x+4}{9} , cross-multiplying gives: 9(8+x)=3(x+4) 9(-8+x) = 3(x+4) , which leads to the same answer.

How do I handle the negative sign in -8 + x?

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Keep the negative sign with the 8. When distributing: 3(8+x)=3×(8)+3×x=24+3x 3(-8 + x) = 3 \times (-8) + 3 \times x = -24 + 3x . The negative stays with the 8!

Can I check my answer a different way?

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Yes! Substitute x = 14 into both sides separately: Left side = 8+143=63=2 \frac{-8+14}{3} = \frac{6}{3} = 2 . Right side = 14+49=189=2 \frac{14+4}{9} = \frac{18}{9} = 2 . Both equal 2! ✓

Why don't we get a fraction answer like the other choices?

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Not all fraction equations have fraction answers! The equation structure determines the solution type. Here, the algebra naturally simplifies to a whole number: x = 14.

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