Solve the Triangle Equation: x with Constraint x > 0

Right Triangles with Quadratic Solutions

Find x.
x>0 x>0

xxxx+7x+7x+7131313

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Find X
00:03 Use the Pythagorean theorem in the triangle
00:06 Substitute appropriate values and solve for X
00:11 Expand brackets correctly
00:21 Collect terms and arrange the equation
00:47 Find the possible solutions for X
00:55 X must be greater than 0, therefore this solution is not logical
00:59 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

Find x.
x>0 x>0

xxxx+7x+7x+7131313

2

Step-by-step solution

To solve this problem, we'll use the Pythagorean Theorem to establish a relationship between the sides of the right triangle:

Given:

  • One side a=x a = x
  • Another side b=x+7 b = x + 7
  • The hypotenuse c=13 c = 13

According to the Pythagorean Theorem:

a2+b2=c2 a^2 + b^2 = c^2

Substitute the given values:

x2+(x+7)2=132 x^2 + (x + 7)^2 = 13^2

Expand and simplify:\

x2+(x2+14x+49)=169 x^2 + (x^2 + 14x + 49) = 169

2x2+14x+49=169 2x^2 + 14x + 49 = 169

Subtract 169 from both sides to set the equation to 0:

2x2+14x+49169=0 2x^2 + 14x + 49 - 169 = 0

2x2+14x120=0 2x^2 + 14x - 120 = 0

Divide the entire equation by 2 to simplify:

x2+7x60=0 x^2 + 7x - 60 = 0

We now have a quadratic equation that can be factored as:

(x+12)(x5)=0 (x + 12)(x - 5) = 0

Set each factor equal to 0 and solve for x x :

  • x+12=0 x + 12 = 0 gives x=12 x = -12
  • x5=0 x - 5 = 0 gives x=5 x = 5

Since x>0 x > 0 , we have x=5 x = 5 .

Therefore, the solution to the problem is x=5 x = 5 .

3

Final Answer

x=5 x=5

Key Points to Remember

Essential concepts to master this topic
  • Pythagorean Theorem: For right triangles, a2+b2=c2 a^2 + b^2 = c^2 always applies
  • Expansion Technique: (x+7)2=x2+14x+49 (x + 7)^2 = x^2 + 14x + 49 using FOIL method
  • Constraint Check: Verify x>0 x > 0 by rejecting x=12 x = -12

Common Mistakes

Avoid these frequent errors
  • Forgetting to apply the constraint x > 0
    Don't accept both solutions x = 5 and x = -12 without checking constraints = impossible negative side length! A triangle cannot have negative side lengths. Always check which solutions satisfy the given constraints.

Practice Quiz

Test your knowledge with interactive questions

Look at the triangle in the diagram. How long is side AB?

222333AAABBBCCC

FAQ

Everything you need to know about this question

Why can't x be negative in this triangle problem?

+

In geometry, side lengths must always be positive! A negative value like x = -12 would mean a side has negative length, which is impossible in real triangles.

How do I know which sides are the legs and which is the hypotenuse?

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The hypotenuse is always the longest side in a right triangle. Since 13 is larger than both x and x+7 (when x=5), the side labeled 13 must be the hypotenuse.

What if I get the wrong factorization for the quadratic?

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Try finding two numbers that multiply to -60 and add to 7. Those numbers are 12 and -5, giving us (x+12)(x5)=0 (x + 12)(x - 5) = 0 .

Should I always expand (x + 7)² by hand?

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Yes! Using the formula (a+b)2=a2+2ab+b2 (a + b)^2 = a^2 + 2ab + b^2 helps avoid mistakes. For (x+7)2 (x + 7)^2 : x² + 2(x)(7) + 7² = x² + 14x + 49.

How can I verify my answer is correct?

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Substitute x = 5 back into the Pythagorean theorem: 52+122=25+144=169=132 5^2 + 12^2 = 25 + 144 = 169 = 13^2 ✓. The equation balances perfectly!

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