# ACT Math : How to find an angle of a line

## Example Questions

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### Example Question #1 : Lines

Two angles are supplementary and have a ratio of 1:4.  What is the size of the smaller angle?

Explanation:

Since the angles are supplementary, their sum is 180 degrees.  Because they are in a ratio of 1:4, the following expression could be written:

### Example Question #1 : Plane Geometry

AB and CD are two parrellel lines intersected by line EF. If the measure of angle 1 is , what is the measure of angle 2?

Explanation:

The angles are equal. When two parallel lines are intersected by a transversal, the corresponding angles have the same measure.

### Example Question #2 : Plane Geometry

Lines A and B in the diagram below are parallel. The triangle at the bottom of the figure is an isosceles triangle.

What is the degree measure of angle ?

Explanation:

Since A and B are parallel, and the triangle is isosceles, we can use the complementary rule for the two angles,  and  which will sum up to . Setting up an algebraic equation for this, we get . Solving for , we get . With this, we can get either  (for the smaller angle) or  (for the larger angle - must then use complimentary rule again for inner smaller angle). Either way, we find that the inner angles at the top are 80 degrees each. Since the sum of the angles within a triangle must equal 180, we can set up the equation as

degrees.

### Example Question #1 : How To Find An Angle Of A Line

In the figure above, lines AB and CD are parallel. Also, a, b, c, d, e, f, g, and h represent the measures of the angles in which they are shown. All of the following must be true EXCEPT:

180 – a – c = e + g – 180

f + d = a + g

f = b

b – h = e – c

a + h = e + d

b – h = e – c

Explanation:

Lines AB and CD are parallel, so that means that d and e, which are in alternate interior angles, must be equal. Because a and d are vertical angles, and because e and h are vertical angles, this means that a = d and e = h. Therefore, a = d = e = h. Similarly, b = c = f = g.

Let us look at the choice f = b. We can see that this is true because b, c, f, and g are all equal.

We can then look at f + d = a + g. Let us subtract g from both sides, and then let us subtract d from both sides. This would give us the following equation.

f – g = a – d

Because f and g are equal, f – g = 0. Also, because a and d are the same, a – d = 0. Therefore, f – g = a – d = 0, so this is always true.

Now, let's look at 180 – a – c = e + g - 180. We can rewrite this as:

180 – (a + c) = e + g – 180.

a and c are supplementary, so a + c = 180. Likewise, e + g = 180. We can substitute 180 in for a + c and for e + g.

180 – 180 = 180 – 180 = 0

This means that 180 – a – c = e + g – 180 is indeed always true.

Next, let's examine the choice a + h = e + d. Let us subtract e from both sides and h from both sides. This will give us the following:

a – e = d – h

Because a = e = d = h, we could replace all of the values with a.

a – a = a – a = 0, so this is always true.

The final choice is b – h = e – c. Let us substitute c in for b and h in for e.

c – h = h – c

Let us add c and h to both sides.

2c = 2h

This means that c must equal h for this to be true. However, c does not always have to equal h. We know that c must equal f, and we know that f + h must equal 180. This means that c + h must equal 180. But this doesn't necessarily mean that c must equal h. In fact, this will only be true if c and h are both 90. Therefore, b – h = e – c isn't always true.

The answer is b – h = e – c.

### Example Question #1 : How To Find The Angle Of Two Lines

Figure not drawn to scale.

In the figure above, APB forms a straight line. If the measure of angle APC is eighty-one degrees larger than the measure of angle DPB, and the measures of angles CPD and DPB are equal, then what is the measure, in degrees, of angle CPB?

66

33

50

40

114

66

Explanation:

Let x equal the measure of angle DPB. Because the measure of angle APC is eighty-one degrees larger than the measure of DPB, we can represent this angle's measure as x + 81. Also, because the measure of angle CPD is equal to the measure of angle DPB, we can represent the measure of CPD as x.

Since APB is a straight line, the sum of the measures of angles DPB, APC, and CPD must all equal 180; therefore, we can write the following equation to find x:

x + (x + 81) + x = 180

Simplify by collecting the x terms.

3x + 81 = 180

Subtract 81 from both sides.

3x = 99

Divide by 3.

x = 33.

This means that the measures of angles DPB and CPD are both equal to 33 degrees. The original question asks us to find the measure of angle CPB, which is equal to the sum of the measures of angles DPB and CPD.

measure of CPB = 33 + 33 = 66.

The answer is 66.

### Example Question #1 : Plane Geometry

One-half of the measure of the supplement of angle ABC is equal to the twice the measure of angle ABC. What is the measure, in degrees, of the complement of angle ABC?

18

36

72

54

90

54

Explanation:

Let x equal the measure of angle ABC, let y equal the measure of the supplement of angle ABC, and let z equal the measure of the complement of angle ABC.

Because x and y are supplements, the sum of their measures must equal 180. In other words, x + y = 180.

We are told that one-half of the measure of the supplement is equal to twice the measure of ABC. We could write this equation as follows:

(1/2)y = 2x.

Because x + y = 180, we can solve for y in terms of x by subtracting x from both sides. In other words, y = 180 – x. Next, we can substitute this value into the equation (1/2)y = 2x and then solve for x.

(1/2)(180-x) = 2x.

Multiply both sides by 2 to get rid of the fraction.

(180 – x) = 4x.

Add x to both sides.

180 = 5x.

Divide both sides by 5.

x = 36.

The measure of angle ABC is 36 degrees. However, the original question asks us to find the measure of the complement of ABC, which we denoted previously as z. Because the sum of the measure of an angle and the measure of its complement equals 90, we can write the following equation:

x + z = 90.

Now, we can substitute 36 as the value of x and then solve for z.

36 + z = 90.

Subtract 36 from both sides.

z = 54.

The answer is 54.

### Example Question #1 : How To Find The Angle Of Two Lines

In the diagram, AB || CD. What is the value of a+b?

60°

160°

80°

None of the other answers.

140°

160°

Explanation:

Refer to the following diagram while reading the explanation:

We know that angle b has to be equal to its vertical angle (the angle directly "across" the intersection).  Therefore, it is 20°.

Furthermore, given the properties of parallel lines, we know that the supplementary angle to a must be 40°.  Based on the rule for supplements, we know that a + 40° = 180°.  Solving for a, we get a = 140°.

Therefore, a + b = 140° + 20° = 160°

### Example Question #3 : Plane Geometry

In rectangle ABCD, both diagonals are drawn and intersect at point E.

Let the measure of angle AEB equal x degrees.

Let the measure of angle BEC equal y degrees.

Let the measure of angle CED equal z degrees.

Find the measure of angle AED in terms of x, y, and/or z.

360 – x + y + z

180 – y

180 – 2(x + z)

180 – 1/2(x + z)

180 – (x + y + z)

180 – 1/2(x + z)

Explanation:

Intersecting lines create two pairs of vertical angles which are congruent. Therefore, we can deduce that y = measure of angle AED.

Furthermore, intersecting lines create adjacent angles that are supplementary (sum to 180 degrees). Therefore, we can deduce that x + y + z + (measure of angle AED) = 360.

Substituting the first equation into the second equation, we get

x + (measure of angle AED) + z + (measure of angle AED) = 360

2(measure of angle AED) + x + z = 360

2(measure of angle AED) = 360 – (x + z)

Divide by two and get:

measure of angle AED = 180 – 1/2(x + z)

### Example Question #2 : Plane Geometry

A student creates a challenge for his friend.  He first draws a square, the adds the line for each of the 2 diagonals.  Finally, he asks his friend to draw the circle that has the most intersections possible.

How many intersections will this circle have?

Explanation:

### Example Question #3 : Sat Mathematics

Two pairs of parallel lines intersect:

If A = 135o, what is 2*|B-C| = ?

180°

160°

150°

170°

140°