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

Consider a right-angled triangle, AB = 8 cm and AC = 6 cm.
Calculate the length of side BC.

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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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