Question 27·Hard·Lines, Angles, and Triangles
A transversal intersects two parallel lines, forming four congruent acute angles and four congruent obtuse angles.
The measure of one of the acute angles is .
The sum of the measures of four of the acute angles and one of the obtuse angles is .
What is the value of ?
(Express the answer as an integer)
For transversal-and-parallel-lines questions, first recall which angles are congruent and which are supplementary (forming straight lines that sum to ). Turn each angle into an algebraic expression, then carefully add or equate them as the problem describes. Often you never need to solve for itself; instead, simplify the total expression and directly compare it to the given form (like ) to match coefficients and constants, which saves time and reduces algebra mistakes.
Hints
Relate acute and obtuse angles
When a transversal intersects parallel lines, each acute angle and its neighboring obtuse angle form a straight line. What is the sum of the measures of angles that form a straight line?
Express the obtuse angle in terms of x
You know the measure of one acute angle is . Use the fact that an acute angle and its adjacent obtuse angle are supplementary to write an expression for the obtuse angle.
Add the angles and compare expressions
Find an expression for the sum of four acute angles and one obtuse angle using your expressions. Then set this equal to and compare the constant terms.
Desmos Guide
Set up the sum of the angles in Desmos
In Desmos, type the expression for the sum of four acute angles and one obtuse angle in terms of :
S(x) = 4*(8x - 22) + (180 - (8x - 22))
This represents the total of four acute angles plus one obtuse angle.
Pick a simple x-value and evaluate the sum
Choose an easy value for , for example x = 10. In Desmos, either define x = 10 or just type S(10) to see the numeric value of the sum of the angles for that .
Use the result to find w
For the same -value (for example, ), compute 24*10 in Desmos. Subtract this from the value of S(10):
S(10) - 24*10
The result is the constant amount that remains after removing the part; that constant is the value of . You will get the same result for any other you choose.
Step-by-step Explanation
Understand the angle relationships
When a transversal cuts two parallel lines:
- There are 4 congruent acute angles and 4 congruent obtuse angles.
- Each acute angle and its adjacent obtuse angle form a linear pair, so they are supplementary and add up to .
So, if one acute angle measures , then the obtuse angle next to it must be minus that acute angle.
Write expressions for the angle measures
The measure of one acute angle is given as .
The adjacent obtuse angle is supplementary to it, so its measure is:
Now find the sum of four acute angles and one obtuse angle:
- Four acute angles:
- One obtuse angle:
Total sum:
Match the given expression and solve for w
The problem states that this same sum is also .
So the two expressions represent the same angle sum:
Since the terms are the same, the constants must be equal:
So, the value of is .