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Example Question #2 : How To Find Negative Cosine

What is the cosine of the angle formed between the origin and the point (-6,-11) if that angle is formed with one side of the angle beginning on the -axis and then rotating counter-clockwise to ? Round to the nearest hundredth.

Possible Answers:

-0.88

0.78

0.48

-0.48

0.88

Correct answer:

-0.48

Explanation:

Recall that when you calculate a trigonometric function for an obtuse angle like this, you always use the -axis as your reference point for your angle. (Hence, it is called the "reference angle.")

Now, it is easiest to think of this like you are drawing a little triangle in the third quadrant of the Cartesian plane. It would look like:

Cos611

So, you first need to calculate the hypotenuse. You can do this by using the Pythagorean Theorem, a^2+b^2=c^2 , where and are the lengths of the legs of the triangle and the length of the hypotenuse. Rearranging the equation to solve for , you get:

$c=\sqrt{a^2+b^2}$

Substituting in the given values:

$c=\sqrt{6^2+11^2}=\sqrt{36+121}=\sqrt{157}$

So, the cosine of an angle is:

$\frac{adjacent}{hypotenuse}$ or, for your data, $\frac{6}{\sqrt{157}}$ .

This is approximately 0.47885213068056 . Rounding, this is 0.48 . However, since (-6,-11) is in the third quadrant your value must be negative: -0.48 .

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Example Question #3 : How To Find Negative Cosine

What is the cosine of the angle formed between the origin and the point (-3,7) if that angle is formed with one side of the angle beginning on the -axis and then rotating counter-clockwise to (-3,7) ? Round to the nearest hundredth.

Possible Answers:

-0.61

0.40

-0.94

-0.40

0.94

Correct answer:

-0.40

Explanation:

Recall that when you calculate a trigonometric function for an obtuse angle like this, you always use the -axis as your reference point for your angle. (Hence, it is called the "reference angle.") Now, it is easiest to think of this like you are drawing a little triangle in the second quadrant of the Cartesian plane. It would look like:

Cos37

So, you first need to calculate the hypotenuse:

$h=\sqrt{3^2+7^2}=\sqrt{9+49}=\sqrt{56}$

So, the cosine of an angle is:

$\frac{adjacent}{hypotenuse}$ or, for your data, $\frac{3}{\sqrt{56}}$ .

This is approximately 0.40089186286863 . Rounding, this is 0.40 . However, since (-3,7) is in the second quadrant your value must be negative: -0.40 .

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Example Question #4 : How To Find Negative Cosine

To the nearest , what is the cosine of the angle formed between the origin and (-4, -14) ? Assume a counterclockwise rotation.

Possible Answers:

-.554

-.605

-.332

-.275

-.545

Correct answer:

-.275

Explanation:

If the point to be reached is (-4, -14) , then we may envision a right triangle with sides and , and hypotenuse . The Pythagorean Theorem tells us that a^2 + b^2 = c^2 , so we plug in and find that: 4^2 + 14^2 = c^2 = 212

Thus, $c = \sqrt{212} = 2\sqrt{53}$

Now, SOHCAHTOA tells us that $\textup{cosine}= \frac{\textup{adjacent}}{\textup{hypotenuse}}$ , so we know that:

$cos x^{\circ} = \frac{4}{2\sqrt{53}} \approx 0.2747$

Thus, our cosine is approximately . However, as we are in the third quadrant, cosine must be negative! Therefore, our true cosine is -.275 .

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Example Question #1 : How To Find Negative Cosine

On a grid, what is the cosine of the angle formed between a line from the origin to (-3, 9) and the x-axis?

Possible Answers:

$\textup{None of the answers provided is correct.}$

$\frac{\sqrt{10}}{10}$

$\frac{3\sqrt{10}}{10}$

$-\frac{3\sqrt{10}}{10}$

$-\frac{\sqrt{10}}{10}$

Correct answer:

$-\frac{\sqrt{10}}{10}$

Explanation:

If the point to be reached is (-3, 9) , then we may envision a right triangle with sides and , and hypotenuse . The Pythagorean Theorem tells us that a^2 + b^2 = c^2 , so we plug in and find that: 3^2 + 9^2 = c^2 = 90 .

Thus, $c = \sqrt{90} = 3\sqrt{10}$ .

Now, SOHCAHTOA tells us that $\textup{cosine} = \frac{\textup{adjacent}}{\textup{hypotenuse}}$ , so we know that:

$\textup{cos} x^{\circ} = \frac{3}{3\sqrt{10}} = \frac{1}{\sqrt{10}} = \frac{\sqrt{10}}{10}$

Thus, our cosine is approximately $\frac{\sqrt{10}}{10}$ . However, as we are in the second quadrant, cosine must be negative! Therefore, our true cosine is $-\frac{\sqrt{10}}{10}$ .

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Example Question #1 : How To Solve For A Variable As Part Of A Fraction

Solve for .

$\frac{16}{x + 7} = \frac{2}{6}$

Possible Answers:

Correct answer:

Explanation:

Cross multiply. 96 = 2 (x+7)

Dsitribute. 96 = 2x + 14

Solve for . 96 - 14 = 2x

82 = 2x

41 = x

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Example Question #1 : How To Solve For A Variable As Part Of A Fraction

The quotient of a fraction is 12x . If the numerator is 48x^3 , what is the value of the denominator?

Possible Answers:

.25x

6x^3

4x^2

576x^4

8x^2

Correct answer:

4x^2

Explanation:

Step 1: Set up the equation

$\frac{48x^3}{D}=12x$

Step 2: Solve for D

$\frac{48x^3}{D}=12x \rightarrow 48x^3= 12x \cdot D\rightarrow \frac{48x^3}{12x}=D$

D = 4x^2

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Example Question #2 : How To Solve For A Variable As Part Of A Fraction

Solve for :

$\frac{x+1}{4}-\frac{x}{3}=6$

Possible Answers:

x=26

x=-72

x=-69

x=60

x=42

Correct answer:

x=-69

Explanation:

Solve for x:

$\frac{x+1}{4}-\frac{x}{3}=6$

Step 1: Find the least common denominator, , and adjust the fractions accordingly:

$\left (\frac{3}{3} \right )\cdot \left ( \frac{x+1}{4} \right ) - \left ( \frac{4}{4} \right )\cdot \left ( \frac{x}{3} \right )=6$

$\left ( \frac{3x+3}{12} \right )-\left ( \frac{4x}{12} \right )=6$

Solve for :

$\frac{3x+3-4x}{12}=6$

3x+3-4x=72

-x=69

x=-69

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Example Question #3241 : Sat Mathematics

If $\frac{6}{x}=\frac{9}{19}$ , then what is the value of ?

Possible Answers:

none of these

3/38

9/114

38/3

7/12

Correct answer:

38/3

Explanation:

cross multiply:

(6)(19) = 9x

114=9x

x = 38/3

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Example Question #1 : How To Solve For A Variable As Part Of A Fraction

$\dpi{100} \small \frac{4 }{x} = \frac{2}{25}$

Find x.

Possible Answers:

$\dpi{100} \small \frac{8}{25}$

$\dpi{100} \small 0.25$

$\dpi{100} \small 50$

None

$\dpi{100} \small \frac{25}{8}$

Correct answer:

$\dpi{100} \small 50$

Explanation:

Cross multiply:

$\dpi{100} \small 4 \times 25 = 2x$

$\dpi{100} \small 100 = 2x$

$\dpi{100} \small x = 50$

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Example Question #3 : How To Solve For A Variable As Part Of A Fraction

The numerator of a fraction is the sum of 4 and 5 times the denominator. If you divide the fraction by 2, the numerator is 3 times the denominator. Find the simplified version of the fraction.

Possible Answers:

$\frac{1}{2}$

$\frac{1}{3}$

Correct answer:

Explanation:

Let numerator = N and denominator = D.

According to the first statement,

N = (D x 5) + 4.

According to the second statement, N / 2 = 3 * D.

Let's multiply the second equation by –2 and add itthe first equation:

–N = –6D

+[N = (D x 5) + 4]

–6D + (D x 5) + 4 = 0

–1D + 4 = 0

D = 4

Thus, N = 24.

Therefore, N/D = 24/4 = 6.

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ACT Math : ACT Math

Study concepts, example questions & explanations for ACT Math

All ACT Math Resources

Example Questions

Example Question #2 : How To Find Negative Cosine

Example Question #3 : How To Find Negative Cosine

Example Question #4 : How To Find Negative Cosine

Example Question #1 : How To Find Negative Cosine

Example Question #1 : How To Solve For A Variable As Part Of A Fraction

Example Question #1 : How To Solve For A Variable As Part Of A Fraction

Example Question #2 : How To Solve For A Variable As Part Of A Fraction

Example Question #3241 : Sat Mathematics

Example Question #1 : How To Solve For A Variable As Part Of A Fraction

Example Question #3 : How To Solve For A Variable As Part Of A Fraction

All ACT Math Resources

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