Solve for X in Intersecting Lines: Finding Angles 2X-9 and 5+X

Alternate Interior Angles with Variable Expressions

Calculate X and the marked angles.

5+X2X-9

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Calculate X and the angles themselves
00:03 Vertical angles are equal to each other
00:10 Compare the expressions and solve for X
00:14 Arrange the equation
00:25 Group like terms
00:33 This is the value of X

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

Calculate X and the marked angles.

5+X2X-9

2

Step-by-step solution

Let us solve this step-by-step:

  • Step 1: Identify the angle relationship.
    Since the angles are positioned on opposite sides of the transversal and between the two parallel lines, we can posit that these angles are alternate interior angles. These angles are equal when the lines are parallel.

  • Step 2: Set up the equation.
    Since the alternate interior angles are equal, we set the expressions equal to one another:
    2X9=5+X 2X - 9 = 5 + X

  • Step 3: Solve the equation for X X .
    Subtract X X from both sides to get: 2XX9=5 2X - X - 9 = 5 which simplifies to: X9=5 X - 9 = 5 Add 9 9 to both sides to find: X=14 X = 14

The value of X X calculated is consistent with the nature of the angle relationships in parallel lines cut by a transversal.

Therefore, the solution to the problem is X=14 X = 14 .

3

Final Answer

14

Key Points to Remember

Essential concepts to master this topic
  • Angle Rule: Alternate interior angles are equal when lines are parallel
  • Technique: Set expressions equal: 2X - 9 = 5 + X
  • Check: Substitute X = 14: 2(14) - 9 = 19 and 5 + 14 = 19 ✓

Common Mistakes

Avoid these frequent errors
  • Assuming angles are supplementary instead of equal
    Don't add the expressions to equal 180° like 2X - 9 + 5 + X = 180 = wrong angle relationship! Alternate interior angles are equal, not supplementary. Always identify the angle relationship first, then set equal expressions.

Practice Quiz

Test your knowledge with interactive questions

If one of two corresponding angles is a right angle, then the other angle will also be a right angle.

FAQ

Everything you need to know about this question

How do I know these are alternate interior angles?

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Look for angles on opposite sides of the transversal (crossing line) and between the parallel lines. They form a "Z" pattern when you trace from one angle to the other.

What if the lines aren't parallel?

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If lines aren't parallel, alternate interior angles are not equal! The problem usually tells you or shows parallel marks. When in doubt, assume they're parallel if angle expressions are given.

Why do I set the expressions equal instead of adding them?

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Alternate interior angles are congruent (equal), not supplementary. Only adjacent angles on a line add to 180°. These angles are in different positions, so they're equal.

How do I solve 2X - 9 = 5 + X step by step?

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Move variables to one side: 2XX=5+9 2X - X = 5 + 9 , which gives X=14 X = 14 . Always collect like terms and isolate the variable!

What are the actual angle measures?

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Substitute X = 14: First angle = 2(14)9=19° 2(14) - 9 = 19° , Second angle = 5+14=19° 5 + 14 = 19° . Both angles measure 19 degrees.

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