ACT Math : ACT Math

Study concepts, example questions & explanations for ACT Math

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Example Questions

Example Question #231 : Act Math

A triangle has a perimeter of \(\displaystyle 40\) inches with one side of length \(\displaystyle 12\) inches. If the remaining two sides have lengths in a ratio of \(\displaystyle 3:4\), what is length of the shortest side of the triangle?

Possible Answers:

\(\displaystyle 12\)

\(\displaystyle 14\)

\(\displaystyle 8\)

\(\displaystyle 16\)

\(\displaystyle 10\)

Correct answer:

\(\displaystyle 12\)

Explanation:

The answer is \(\displaystyle 12\).

Since we know that the permieter is \(\displaystyle 40\) inches and one side is \(\displaystyle 12\) inches, it can be determined that the remaining two sides must combine to be \(\displaystyle 40-12=28\) inches. The ratio of the remaining two sides is \(\displaystyle 3:4\) which means 3 parts : 4 parts or 7 parts combined. We can then set up the equation \(\displaystyle 7x=28\), and divide both sides by \(\displaystyle 7\) which means \(\displaystyle x=4\). The ratio of the remaining side lengths then becomes \(\displaystyle 3*(4):4*(4)\) or \(\displaystyle 12:16\). We now know the 3 side lengths are \(\displaystyle 12,12,and\ 16\).

\(\displaystyle 12\) is the shortest side and thus the answer.

Example Question #2 : How To Find The Length Of The Side Of Of An Acute / Obtuse Isosceles Triangle

In the standard \(\displaystyle (x,y)\) coordinate plane, the points \(\displaystyle (1,-1)\) and \(\displaystyle (1,7)\) form two vertices of an isosceles triangle.  Which of the following points could be the third vertex? 

Possible Answers:

\(\displaystyle (2,3)\)

\(\displaystyle (0,0)\)

\(\displaystyle (4,5)\)

\(\displaystyle (3,4)\)

\(\displaystyle (2,4)\)

Correct answer:

\(\displaystyle (2,3)\)

Explanation:

To form an isosceles triangle here, we need to create a third vertex whose \(\displaystyle y\) coordinate is between \(\displaystyle -1\) and \(\displaystyle 7\).  If a vertex is placed at \(\displaystyle (2,3)\), the distance from \(\displaystyle (1,-1)\) to this point will be \(\displaystyle \sqrt{17}\). The distance from \(\displaystyle (1,7)\) to this point will be the same.

Example Question #175 : Triangles

Screen_shot_2015-05-07_at_4.37.54_pm

Note: Figure is not drawn to scale.

In the figure above, points \(\displaystyle A, B, C\) are collinear and \(\displaystyle \angle\) \(\displaystyle ABE\) is a right angle. If \(\displaystyle \overline{BD}=\overline{DE}\) and \(\displaystyle \measuredangle\)\(\displaystyle BDE\) is \(\displaystyle 58^{\circ}\), what is \(\displaystyle \measuredangle CBD\)?  

Possible Answers:

\(\displaystyle 29^{\circ}\)

\(\displaystyle 30^{\circ}\)

\(\displaystyle 61^{\circ}\)

\(\displaystyle 122^{\circ}\)

\(\displaystyle 35^{\circ}\)

Correct answer:

\(\displaystyle 29^{\circ}\)

Explanation:

Because \(\displaystyle \bigtriangleup BDE\) is isosceles, \(\displaystyle \measuredangle DBE\) equals \(\displaystyle \left(\frac{180-58}{2}\right)^{\circ}\) or \(\displaystyle 61^{\circ}\).

We know that \(\displaystyle \measuredangle ABE, \measuredangle DBE, \measuredangle CBD\) add up to \(\displaystyle 180^{\circ}\), so \(\displaystyle \measuredangle CBD\) must equal \(\displaystyle (180-90-61)^{\circ}\) or \(\displaystyle 29^{\circ}\).

Example Question #171 : Triangles

A light beam of pure white light is aimed horizontally at a prism, which splits the light into two streams that diverge at a \(\displaystyle 30^{\circ}\) angle. The split beams each travel exactly \(\displaystyle \textup{5 feet}\) from the prism before striking two optic sensors (one for each beam).

What is the distance, in feet, between the two sensors?

Round your final answer to the nearest tenth. Do not round until then.

Possible Answers:

\(\displaystyle 2.6\)

\(\displaystyle 7.3\)

\(\displaystyle 2.9\)

\(\displaystyle 7.7\)

\(\displaystyle 2.1\)

Correct answer:

\(\displaystyle 2.6\)

Explanation:

This problem can be solved when one realizes that the light beam's split has resulted in an acute isosceles triangle. The triangle as stated has two sides of \(\displaystyle 5\) feet apiece, which meets the requirement for isosceles triangles, and having one angle of \(\displaystyle 30^{\circ}\) at the vertex where the two congruent sides meet means the other two angles must be \(\displaystyle 75^{\circ}\) and \(\displaystyle 75^{\circ}\). The missing side connecting the two sensors, therefore, is opposite the \(\displaystyle 30^{\circ}\) angle.

Since we know at least two angles and at least one side of our triangle, we can use the Law of Sines to calculate the remainder. The Law of Sines says that for any triangle with angles \(\displaystyle A, B\) and \(\displaystyle C,\) and opposite sides \(\displaystyle a, b\) and \(\displaystyle c\):

\(\displaystyle \frac{\sin A}{a} = \frac{\sin B}{b} = \frac{\sin C}{c}\).

Plugging in one of our \(\displaystyle 75^{\circ}\) angles (and its corresponding \(\displaystyle 5\) ft side) into this equation, as well as our \(\displaystyle 30^{\circ}\) angle (and its corresponding unknown side) into this equation gives us:

\(\displaystyle \frac{\sin 75^{\circ}}{5} = \frac{ \sin 30^{\circ}}{x}\)

Next, cross-multiply:

\(\displaystyle \frac{\sin 75^{\circ}}{5} = \frac{\sin 30^{\circ}}{x}\) ---> \(\displaystyle x\sin 75^{\circ} = 5\sin 30^{\circ}\)

Now simplify and solve:

\(\displaystyle x = \frac{5\sin 30^{\circ}}{\sin 75^{\circ}} = 2.588\)

Rounding, we see our missing side is \(\displaystyle 2.65\textup{ feet}\) long.

Example Question #1 : Isosceles Triangles

An isosceles triangle has a base of 12\ cm\(\displaystyle 12\ cm\) and an area of 42\ cm^{2}\(\displaystyle 42\ cm^{2}\). What must be the height of this triangle?

Possible Answers:

6\ cm\(\displaystyle 6\ cm\)

7\ cm\(\displaystyle 7\ cm\)

10\ cm\(\displaystyle 10\ cm\)

9\ cm\(\displaystyle 9\ cm\)

8\ cm\(\displaystyle 8\ cm\)

Correct answer:

7\ cm\(\displaystyle 7\ cm\)

Explanation:

A=\frac{1}{2}bh\(\displaystyle A=\frac{1}{2}bh\)

6x=42\(\displaystyle 6x=42\)

x=7\(\displaystyle x=7\)

Example Question #231 : Plane Geometry

What is the height of an isosceles triangle which has a base of \(\displaystyle 4cm\) and an area of \(\displaystyle 24cm^2\)?

Possible Answers:

\(\displaystyle 16cm\)

\(\displaystyle 12cm\)

\(\displaystyle 24cm\)

\(\displaystyle 8cm\)

\(\displaystyle 6cm\)

Correct answer:

\(\displaystyle 12cm\)

Explanation:

The area of a triangle is given by the equation: 

\(\displaystyle \small A=\frac{1}{2}bh\)

In this case, we are given the area (\(\displaystyle \small A\)) and the base (\(\displaystyle \small b\)) and are asked to solve for height (\(\displaystyle \small h\)).

To do this, we must plug in the given values for \(\displaystyle \small A\) and \(\displaystyle \small b\)which gives the following:

\(\displaystyle \small 24\;cm^2=\frac{1}{2}(4\; cm)h\)

We then must multiply the right side, and then divide the entire equation by 2, in order to solve for \(\displaystyle \small h\):

\(\displaystyle \small 24\;cm^2=2\;cm(h)\)

\(\displaystyle \small \frac{24\; cm^2}{2\; cm}=\frac{2\; cm(h)}{2\; cm}\)

\(\displaystyle \small 12\;cm=h\)

\(\displaystyle \small h=12\;cm\)

Therefore, the height of the triangle is \(\displaystyle \small 12\;cm\).

Example Question #41 : Isosceles Triangles

There are two obtuse triangles. The obtuse angle of triangle one is \(\displaystyle 101^{\circ}\). The sum of two angles in the second triangle is \(\displaystyle 80^{\circ}\). When are these two triangles congruent?

Possible Answers:

When the sum of angle A and angle B in triangle 1 is equal to the sum of the corresponding angles in triangle 2

When the obtuse angle is congruent to the smallest angle of the other triangle

The two triangles must be congruent

The two triangles cannot be congruent

Cannot be determined

Correct answer:

The two triangles cannot be congruent

Explanation:

In order for two obtuse triangles to be congruent, the sum of the two smaller angles must equal the sum of the two smaller angles of the second triangle. That is, excluding the obtuse angle. 

The first triangle has an obtuse angle of \(\displaystyle 101^{\circ}\). That means the sum of the other two angles is \(\displaystyle 79^{\circ}\). The sum of the corresponding angles in triangle 2 is \(\displaystyle 80^{\circ}\). Therefore, because \(\displaystyle 79^{\circ}\) is not equal to \(\displaystyle 80^{\circ}\), the two obtuse triangles cannot be congruent.

Example Question #1 : How To Find An Angle In A Kite

Kite vt act

Using the kite shown above, find the sum of the two remaining congruent interior angles. 

Possible Answers:

\(\displaystyle 295$^{\circ}$\)

\(\displaystyle 300$^{\circ}$\)

\(\displaystyle 75$^{\circ}$\)

\(\displaystyle 300.5$^{\circ}$\)

\(\displaystyle 147.5$^{\circ}$\)

Correct answer:

\(\displaystyle 295$^{\circ}$\)

Explanation:

The sum of the interior angles of any polygon can be found by applying the formula: 

\(\displaystyle 180 (n-2)\) degrees, where \(\displaystyle n\) is the number of sides in the polygon. 

By definition, a kite is a polygon with four total sides (quadrilateral). The sum of the interior angles of any quadrilateral must equal: \(\displaystyle 180(4-2)\) degrees \(\displaystyle =180(2)\) degrees \(\displaystyle =360\) degrees. Additionally, kites must have two sets of equivalent adjacent sides & one set of congruent opposite angles

To find the sum of the remaining two angles, determine the difference between \(\displaystyle 360\) degrees and the sum of the non-congruent opposite angles.

The solution is:

\(\displaystyle 40+25=65\)

\(\displaystyle 360-65=295\) degrees

Thus, \(\displaystyle 295\) degrees is the sum of the remaining two opposite angles.

Check:

\(\displaystyle 295+65=360\)

Example Question #1 : How To Find An Angle In A Kite

A kite has one set of opposite interior angles where the two angles measure \(\displaystyle 28$^{\circ}$\) and \(\displaystyle 84$^{\circ}$\), respectively. Find the measurement for one of the two remaining interior angles in this kite. 

Possible Answers:

\(\displaystyle 112$^{\circ}$\)

\(\displaystyle 248$^{\circ}$\)

\(\displaystyle 124$^{\circ}$\)

\(\displaystyle 84^2$^{\circ}$\)

Not enough information is provided 

Correct answer:

\(\displaystyle 124$^{\circ}$\)

Explanation:

The sum of the interior angles of any polygon can be found by applying the formula:

\(\displaystyle 180 (n-2)\) degrees, where \(\displaystyle n\) is the number of sides in the polygon.

By definition, a kite is a polygon with four total sides (quadrilateral). The sum of the interior angles of any quadrilateral must equal: \(\displaystyle 180(4-2)\) degrees \(\displaystyle =180(2)\) degrees \(\displaystyle =360\) degrees. Additionally, kites must have two sets of equivalent adjacent sides & one set of congruent opposite angles.

The missing angle can be found by finding the sum of the non-congruent opposite angles. Then divide the difference between \(\displaystyle 360\) degrees and the non-congruent opposite angles sum by \(\displaystyle 2\):   

\(\displaystyle 28+84=112\)

\(\displaystyle 360-112=248\)

This means that \(\displaystyle 248\) is the sum of the remaining two angles, which must be opposite congruent angles. Therefore, the measurement for one of the angles is: 

\(\displaystyle \frac{248}{2}=124\) 

Example Question #1 : How To Find An Angle In A Kite

A kite has one set of opposite interior angles where the two angles measure \(\displaystyle 118$^{\circ}$\) and \(\displaystyle 51$^{\circ}$\), respectively. Find the measurement for one of the two remaining interior angles in this kite. 

Possible Answers:

\(\displaystyle 59.5$^{\circ}$\)

\(\displaystyle 95.5$^{\circ}$\)

\(\displaystyle 169$^{\circ}$\)

\(\displaystyle 95$^{\circ}$\)

\(\displaystyle 191$^{\circ}$\)

Correct answer:

\(\displaystyle 95.5$^{\circ}$\)

Explanation:

The sum of the interior angles of any polygon can be found by applying the formula: 

\(\displaystyle 180 (n-2)\) degrees, where \(\displaystyle n\) is the number of sides in the polygon. 

A kite is a polygon with four total sides (quadrilateral). The sum of the interior angles of any quadrilateral must equal: \(\displaystyle 180(4-2)\) degrees \(\displaystyle =180(2)\) degrees \(\displaystyle =360\) degrees. Additionally, kites must have two sets of equivalent adjacent sides & one set of congruent opposite angles

The missing angle can be found by finding the sum of the non-congruent opposite angles. Then divide the difference between \(\displaystyle 360\) degrees and the non-congruent opposite angles sum by \(\displaystyle 2\):   

\(\displaystyle 118+51=169\)

\(\displaystyle 360-169=191\)

This means that \(\displaystyle 191\) is the sum of the remaining two angles, which must be opposite congruent angles. Therefore, the measurement for one of the angles is: 

\(\displaystyle \frac{191}{2}=95.5\) 

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