Equation of a Straight Line: Using the angle formed by the line with the axes

Examples with solutions for Equation of a Straight Line: Using the angle formed by the line with the axes

Exercise #1

Two lines haves slopes of -3 and -6.

Which of the lines forms a greater angle with the x axis?

Video Solution

Step-by-Step Solution

To find which line forms a greater angle with the x-axis, we use the formula θ=tan⁡−1(m) \theta = \tan^{-1}(m) . This gives us the angles formed by lines with slopes m1=−3 m_1 = -3 and m2=−6 m_2 = -6 .

We first calculate the inverse tangent for both slopes:

  • For m1=−3 m_1 = -3 , the angle is θ1=tan⁡−1(−3) \theta_1 = \tan^{-1}(-3) .
  • For m2=−6 m_2 = -6 , the angle is θ2=tan⁡−1(−6) \theta_2 = \tan^{-1}(-6) .

The greater the absolute value of the negative slope, the closer the line is to the vertical, thus forming a greater angle with the x-axis.

Comparing ∣−3∣ |-3| and ∣−6∣ |-6| , we see that −6-6 has a greater absolute value, indicating a steeper angle. Hence, although −6-6 is steeper, it forms a smaller angle with the x-axis because we're considering angles formed in the positive x-direction (i.e., above the x-axis).

Therefore, the line whose slope is −3-3 forms the greater angle with the x-axis.

The line whose slope is -3 forms the greater angle.

Answer

The line whose slope is -3 forms the greater angle.

Exercise #2

Two straight lines have slopes of2,12 2,\frac{1}{2} .

Which of the lines forms a larger angle with the x axis?

Video Solution

Step-by-Step Solution

To solve for which line forms a larger angle with the x-axis, we'll proceed by calculating the arctangent of each slope:

  • First, calculate the angle for the line with slope m1=2 m_1 = 2 :

θ1=tan⁡−1(2) \theta_1 = \tan^{-1}(2)

  • Next, calculate the angle for the line with slope m2=12 m_2 = \frac{1}{2} :

θ2=tan⁡−1(12) \theta_2 = \tan^{-1}\left(\frac{1}{2}\right)

Comparing these two results:

  • The function tan⁡−1(x) \tan^{-1}(x) is increasing, meaning as the value of x x increases, so does the angle θ \theta . Therefore, since 2>12 2 > \frac{1}{2} , it follows that θ1>θ2 \theta_1 > \theta_2 .

Therefore, the straight line with a slope of 2 forms the largest angle with the x-axis.

Answer

The straight line with a slope of 2 forms the largest angle.

Exercise #3

Two lines have slopes of −6 -6 and 12 \frac{1}{2} .

Which of the lines forms a smaller angle with the x-axis?

Video Solution

Step-by-Step Solution

We will use the formula:

m=tan⁡α m=\tan\alpha

Let's check the slope of minus 6:

−6=tan⁡α -6=\tan\alpha

tan⁡−1(−6)=α \tan^{-1}(-6)=\alpha

−80.53=α -80.53=\alpha

180−80.53= 180-80.53=

99.47=α1 99.47=\alpha_1

Let's check the slope of one-half:

12=tan⁡α \frac{1}{2}=\tan\alpha

tan⁡−1(12)=α \tan^{-1}(\frac{1}{2})=\alpha

26.56=α2 26.56=\alpha_2

α1>α2 \alpha_1 > \alpha_2

Answer

The line with a slope of 12 \frac{1}{2}

Exercise #4

Which algebraic equation represents a straight line that passes through the point (3,14) (3,14) and creates an angle of 135 degrees with the positive part of the x axis?

Video Solution

Step-by-Step Solution

To solve this problem, we'll follow these steps:

  • Step 1: Calculate the slope
  • Step 2: Apply the point-slope form to get the line equation
  • Step 3: Simplify and verify against given choices

Now, let's work through each step:
Step 1: The slope for a line making 135 degrees with the x-axis is m=tan⁡(135∘)=tan⁡(135∘−180∘)=tan⁡(−45∘)=−1 m = \tan(135^\circ) = \tan(135^\circ - 180^\circ) = \tan(-45^\circ) = -1 .
Step 2: Using the point-slope formula y−y1=m(x−x1) y - y_1 = m(x - x_1) with (x1,y1)=(3,14)(x_1, y_1) = (3, 14) and m=−1 m = -1 , we have:
y−14=−1(x−3) y - 14 = -1(x - 3) .
Simplifying, y−14=−x+3 y - 14 = -x + 3 .
Rearranging terms gives y=−x+17 y = -x + 17 , or equivalently y+x=17 y + x = 17 .

Therefore, the equation of the line is y+x=17 y + x = 17 .

Answer

y+x=17 y+x=17

Exercise #5

Choose the equation that represents a straight line that passes through the point (2,2) (2,2) and creates an angle of 180 degrees with the positive part of the x axis.

Video Solution

Step-by-Step Solution

To solve this problem, we'll find the equation of a line passing through (2,2) (2,2) that makes an angle of 180∘ 180^\circ with the positive x-axis.

  • Step 1: Calculate the Slope.
    The slope m m of the line can be found using the tangent of the angle. The angle given is 180∘ 180^\circ .
    Therefore, m=tan⁡(180∘)=0 m = \tan(180^\circ) = 0 .
    This tells us that the line is horizontal.
  • Step 2: Apply the Point-Slope Form.
    Since the line is horizontal (slope = 0), it has a constant y y -value.
    The point given is (2,2) (2, 2) , meaning the line's equation is y=2 y = 2 .

Thus, the equation of the line is y=2 y = 2 . This corresponds to choice 4 4 in the given options, confirming that it meets all criteria of the problem.

Therefore, the solution to the problem is y=2 y = 2 .

Answer

y=2 y=2

Exercise #6

Find the slope of a line that makes an angle of 100 degrees with the positive part of the xaxis, and indicate whether the line is ascending or descending.

Video Solution

Step-by-Step Solution

To solve this problem, we'll follow these steps:

  • Step 1: Use the formula m=tan⁡(θ) m = \tan(\theta)
  • Step 2: Calculate tan⁡(100∘)\tan(100^\circ)
  • Step 3: Determine if the line is ascending or descending based on the slope

Now, let's work through each step:

Step 1: We use the formula for the slope of a line:
m=tan⁡(θ)\displaystyle m = \tan(\theta)

Step 2: Substitute θ=100∘\theta = 100^\circ into the formula:
m=tan⁡(100∘)\displaystyle m = \tan(100^\circ)

Using a calculator, tan⁡(100∘)≈−5.67\tan(100^\circ) \approx -5.67.

Step 3: Since the slope is negative, the line is descending.

Therefore, the solution to the problem is: m=−5.67 m = -5.67 , decreasing.

Answer

m=−5.67 m=-5.67 decreasing

Exercise #7

Which function represents a straight line that passes through the point (−3,−5) (-3,-5) and creates an angle of 45 degrees with the positive part of the x axis?

Video Solution

Step-by-Step Solution

To solve this problem, let's start by finding the slope mm of the line. Since the line makes a 45-degree angle with the positive xx-axis, the slope m=tan⁡(45∘)=1m = \tan(45^\circ) = 1.

Next, we will use the point-slope form of the line equation, which is given by:

y−y1=m(x−x1) y - y_1 = m(x - x_1)

where (x1,y1)=(−3,−5)(x_1, y_1) = (-3, -5) and m=1m = 1. Substituting these values, we have:

y+5=1⋅(x+3) y + 5 = 1 \cdot (x + 3)

which simplifies to:

y+5=x+3 y + 5 = x + 3

Subtract 5 from both sides to put it into slope-intercept form y=mx+by = mx + b, we get:

y=x−2 y = x - 2

Therefore, the function that represents the line through (−3,−5)(-3, -5) with a 45-degree angle with the xx-axis is y=x−2 y = x - 2 .

Answer

y=x−2 y=x-2

Exercise #8

Choose the algebraic equation that describes a straight line that passes through the point (−9,2) (-9,2) and forms a 30-degrees angle with the positive part of the x axis.

Video Solution

Answer

3y−3x=6+93 3y-\sqrt{3x}=6+9\sqrt{3}