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 ?
A linear pair is two neighboring angles that make a straight line, so their measures add to . When a transversal crosses parallel lines, an acute angle and an obtuse angle form such a pair. In a sum containing several acute angles and one obtuse angle, group one acute angle with the obtuse angle first. Then match the resulting expression to the given total; don't count the obtuse angle as another acute angle.
Hints
- Hint 1
At either crossing, an acute angle sits beside an obtuse angle. Those two angles form a linear pair, so their measures add to . How could you use that pair in the sum?
- Hint 2
The four acute angles have the same measure. Set one aside to pair with the obtuse angle. How many acute angles remain outside the pair?
- Hint 3
The two expressions for the total must agree for every valid value of . In Desmos, a regression can find the constant that makes them agree at several values.
Step-by-step
Approach 1: Group a straight angle, then match the totals
Step 1Relate the acute and obtuse angles
Let be the measure of each acute angle, and let be the measure of an obtuse angle. At either crossing, an acute angle and an obtuse angle beside it form a linear pair: together they make a straight angle. So .
- Step 2
Write the total in two ways
The problem counts four acute angles and one obtuse angle. Using and for their measures, that total is . It is also given as , so
- Step 3
Set aside one acute angle
Break the four copies of into three copies and one more. That puts one acute angle beside the obtuse angle in the equation:
- Step 4
Replace the pair with a straight angle
Since , trade the acute-obtuse pair for : . The replaces both angles in the pair, not the obtuse angle alone.
- Step 5
Use the given acute-angle measure
Substitute into the total: . This equality must hold for every valid , because both sides describe the same five-angle sum.
- Step 6
Find the constant with Desmos
Type , then . The list gives Desmos several inputs where is acute; asks it to fit the constant so the totals match. Under PARAMETERS, Desmos shows . So the value of is . Grid in 114.
Approach 2: See why the constant matches
Step 1Expand the remaining acute angles
Instead of fitting , compare the parts of with . Distribute : . The terms already match.
- Step 2
Remove the matching terms
Subtract from both sides: . So comes from the constant parts of the acute angles and the straight angle.
- Step 3
Finish the constant
Add the constants: . So . Grid in 114.