SAT Math : Plane Geometry

Study concepts, example questions & explanations for SAT Math

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Example Questions

Example Question #142 : Triangles

The base angle of an isosceles triangle is 27^{\circ}.  What is the vertex angle?

Possible Answers:

126^{\circ}

108^{\circ}

149^{\circ}

75^{\circ}

135^{\circ}

Correct answer:

126^{\circ}

Explanation:

Every triangle has 180 degrees.  An isosceles triangle has one vertex angle and two congruent base angles. 

Solve the equation 27+27+x=180 for x to find the measure of the vertex angle. 

x = 180 - 27 - 27

x = 126

Therefore the measure of the vertex angle is 126^{\circ}.

Example Question #143 : Triangles

An isosceles triangle has an area of 12. If the ratio of the base to the height is 3:2, what is the length of the two equal sides?

 

Possible Answers:

4√3

4

5

6

3√3

Correct answer:

5

Explanation:

Area of a triangle is ½ x base x height. Since base:height = 3:2, base = 1.5 height.  Area = 12 = ½ x 1.5 height x height or 24/1.5 = height2.  Height = 4.  Base = 1.5 height = 6. Half the base and the height form the legs of a right triangle, with an equal leg of the isosceles triangle as the hypotenuse. This is a 3-4-5 right triangle.

 Sat_math_167_01

 

 

 

 

Example Question #171 : Plane Geometry

Two sides of a triangle each have length 6. All of the following could be the length of the third side EXCEPT

Possible Answers:
11
12
2
3
1
Correct answer: 12
Explanation:

This question is about the Triangle Inequality, which states that in a triangle with two sides A and B, the third side must be greater than the absolute value of the difference between A and B and smaller than the sum of A and B.

Applying the Triangle Inequality to this problem, we see that the third side must be greater than the absolute value of the difference between the other two sides, which is |6-6|=0, and smaller than the sum of the two other sides, which is 6+6=12. The only answer choice that does not satisfy this range of possible values is 12 since the third side must be LESS than 12.

 

Example Question #146 : Triangles

A triangle has the following side lengths:

Which of the following correctly describes the triangle? 

Possible Answers:

None of these

Obtuse and scalene

Obtuse and isosceles

Acute and scalene

Acute and isosceles

Correct answer:

Acute and isosceles

Explanation:

The triangle has two sides of equal length, 13, so it is by definition isosceles. 

To determine whether the triangle is acute, right, or obtuse, compare the sum of the squares of the lengths of the two shortest sides to the square of the length of the longest side. The former quantity is equal to

The latter quantity is equal to

The former is greater than the latter; consequently, the triangle is acute. The correct response is that the triangle is acute and isosceles.

Example Question #1 : How To Find The Area Of A Trapezoid

A trapezoid has a base of length 4, another base of length s, and a height of length s. A square has sides of length s. What is the value of s such that the area of the trapezoid and the area of the square are equal?

Possible Answers:

Correct answer:

Explanation:

In general, the formula for the area of a trapezoid is (1/2)(a + b)(h), where a and b are the lengths of the bases, and h is the length of the height. Thus, we can write the area for the trapezoid given in the problem as follows:

area of trapezoid = (1/2)(4 + s)(s)

Similarly, the area of a square with sides of length a is given by a2. Thus, the area of the square given in the problem is s2.

We now can set the area of the trapezoid equal to the area of the square and solve for s.

(1/2)(4 + s)(s) = s2

Multiply both sides by 2 to eliminate the 1/2.

(4 + s)(s) = 2s2

Distribute the s on the left.

4s + s2 = 2s2

Subtract s2 from both sides.

4s = s2

Because s must be a positive number, we can divide both sides by s.

4 = s

This means the value of s must be 4.

The answer is 4.

Example Question #1 : Trapezoids

Find the area of a trapezoid given bases of length 1 and 2 and height of 2.

Possible Answers:

Correct answer:

Explanation:

To solve, simply use the formula for the area of a trapezoid. Thus,

 

Example Question #2 : Trapezoids

Square 1

The above figure shows Square is the midpoint of  is the midpoint of  is the midpoint of . Construct 

If Square  has area , what is the area of Quadrilateral ?

Possible Answers:

Correct answer:
Explanation:

Let  be the common sidelength of the square. The area of the square is .

Construct segment . This divides the square into Rectangle  and a right triangle. The dimensions of Rectangle  are 

and

.

 

The area of Rectangle  s the product of these dimensions:

The lengths of the legs of Right  are

and 

The area of this right triangle is half the product of these lengths, or

This is seen below:

Square 2

The sum of these areas is the area of Quadrilateral 

.

Substituting  for , the area is .

Example Question #3 : Quadrilaterals

Square 1

The above figure shows Square is the midpoint of  is the midpoint of  is the midpoint of . Construct 

. Which of the following expresses the length of   in terms of ?

Possible Answers:

Correct answer:

Explanation:

Construct  as shown in the diagram below:

Square 1

Quadrilateral  is a rectangle, so opposite sides are congruent. Therefore, .

Since  is the midpoint of ,

Since  is the midpoint of 

.

 is a right triangle, so, by the Pythagorean Theorem, 

Substituting: 

Apply the Product of Radicals and Quotient of Radicals Rules:

 

Example Question #2 : Quadrilaterals

ABCD is a parallelogram. BD = 5. The angles of triangle ABD are all equal. What is the perimeter of the parallelogram?

Figure_1

Possible Answers:

Correct answer:

Explanation:

If all of the angles in triangle ABD are equal and line BD divides the parallelogram, then all angles in triangle BDC must be equal as well.

We now have two equilateral triangles, so all sides of the triangles will be equal.

All sides therefore equal 5.

5+5+5+5 = 20

Example Question #1 : How To Find The Area Of A Parallelogram

Find the area of a parallelogram with length 6 and height 5.

Possible Answers:

Correct answer:

Explanation:

To solve, simply use the formula for the area of a parallelogram. Thus,

Don't let a parallelogram fool you. It is just like a rectangle but on a slant. As long as you are given the height, and not the slant side length, you can use the same formula.

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