Solve -4(x²+5) = (-x+7)(4x-9)+5: Complete Quadratic Solution

Quadratic Simplification with Linear Result

4(x2+5)=(x+7)(4x9)+5 -4(x^2+5)=(-x+7)(4x-9)+5

x=? x=?

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Solve
00:04 Open parentheses properly multiply by each factor:
00:12 Open parentheses properly multiply each factor by each factor:
00:37 Collect terms
00:44 We want to isolate the unknown X
00:49 Simplify what we can
01:01 Arrange the equation so that only X is on one side
01:13 Isolate the unknown X
01:24 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

4(x2+5)=(x+7)(4x9)+5 -4(x^2+5)=(-x+7)(4x-9)+5

x=? x=?

2

Step-by-step solution

To solve this equation, we'll follow these steps:

  • Step 1: Expand and simplify the right-hand side.
  • Step 2: Set the equation to zero by moving all terms to one side.
  • Step 3: Simplify to obtain a standard quadratic equation.
  • Step 4: Use the quadratic formula to find the possible solutions for x x .

Now, let's work through each step:

Step 1:
Expand the right-hand side:
(x+7)(4x9)=x(4x)x(9)+7(4x)7(9)(-x + 7)(4x - 9) = -x(4x) - x(-9) + 7(4x) - 7(9)
= 4x2+9x+28x63-4x^2 + 9x + 28x - 63
Considering both sides: 4(x2+5)=4x2+9x+28x63+5 -4(x^2 + 5) = -4x^2 + 9x + 28x - 63 + 5 .

Step 2:
Simplify further by calculating:
4x220=4x2+37x58-4x^2 - 20 = -4x^2 + 37x - 58.

Step 3:
Move all terms to one side to achieve zero on the right-hand side:
4x220+4x237x+58=0-4x^2 - 20 + 4x^2 - 37x + 58 = 0
Simplifying, we get: 37x+38=037x + 38 = 0.

Step 4:
Since the x2 x^2 terms cancel, it's actually a linear equation:
37x=38 37x = -38 .
Solving for x x , we divide both sides by 37:
x=3837=1137 x = \frac{-38}{37} = -1\frac{1}{37} .

Therefore, the solution to the problem is x=1137 x = 1\frac{1}{37} .

3

Final Answer

1137 1\frac{1}{37}

Key Points to Remember

Essential concepts to master this topic
  • Expansion: Distribute carefully and combine like terms systematically
  • Technique: (-x + 7)(4x - 9) = -4x² + 9x + 28x - 63
  • Check: Substitute x = 1 1/37 back into original equation ✓

Common Mistakes

Avoid these frequent errors
  • Not recognizing when quadratic terms cancel
    Don't assume every equation with x² terms will be quadratic = missing linear solutions! When x² terms cancel completely, you get a linear equation with one solution. Always simplify completely before identifying equation type.

Practice Quiz

Test your knowledge with interactive questions

Solve for X:

\( 3x=18 \)

FAQ

Everything you need to know about this question

Why did this become a linear equation instead of quadratic?

+

When you expand both sides, the x² terms cancel out! The left side has -4x² and the right side also has -4x², so they eliminate each other, leaving only linear terms.

How do I know if I expanded correctly?

+

Use FOIL for (-x + 7)(4x - 9): First terms (-x)(4x) = -4x², Outer terms (-x)(-9) = 9x, Inner terms (7)(4x) = 28x, Last terms (7)(-9) = -63.

What's the difference between 1 1/37 and -1 1/37?

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Be careful with signs! x=3837=1137 x = \frac{-38}{37} = -1\frac{1}{37} is negative, but the correct answer is 1137 1\frac{1}{37} which is positive. Double-check your arithmetic!

Should I use the quadratic formula here?

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No! Since the x² terms canceled out, this is actually a linear equation. The quadratic formula only applies when you have ax2+bx+c=0 ax^2 + bx + c = 0 with a ≠ 0.

How can I avoid sign errors when expanding?

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Write out each multiplication step clearly: (-x + 7)(4x - 9). Use the distributive property systematically and keep track of negative signs at each step.

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