Find the Perpendicular Line: Solving 3y=4x-y through Point (-3,-15)

Perpendicular Lines with Point-Slope Form

Which function represents a straight line that passes through the point (3,15) (-3,-15) and is perpendicular to the line
3y=4xy 3y=4x-y ?

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Find the algebraic representation of the function
00:03 Isolate Y
00:13 This is the equation of the perpendicular function
00:20 This is the slope of the perpendicular function
00:24 The product of slopes of perpendicular lines is (-1)
00:32 Substitute the slope and solve to find the second slope
00:42 This is the slope of our function
00:47 A point through which our function passes according to the given data
00:50 We'll use the line equation
00:54 Substitute the point according to the given data
00:59 Substitute the slope, and solve to find the intersection point (B)
01:15 Isolate the intersection point (B)
01:21 This is the intersection point with the Y-axis
01:24 Now let's substitute the intersection point and slope in the line equation
01:44 Arrange the equation
01:50 And this is the solution to the problem

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

Which function represents a straight line that passes through the point (3,15) (-3,-15) and is perpendicular to the line
3y=4xy 3y=4x-y ?

2

Step-by-step solution

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

  • Step 1: Rearrange the given line equation to standard slope-intercept form.
  • Step 2: Determine the slope of the perpendicular line.
  • Step 3: Use the point-slope form to find the equation of the desired line.

Now, let's work through each step:

Step 1: The given line equation is 3y=4xy3y = 4x - y. First, simplify this equation:

3y=4xy 3y = 4x - y

Add yy to both sides to consolidate yy:

3y+y=4x 3y + y = 4x

4y=4x 4y = 4x

Divide both sides by 4:

y=x y = x

The slope mm of this line is 1.

Step 2: Since the line we want is perpendicular to this line, the slope m1m_1 of the desired line should satisfy:

m×m1=1 m \times m_1 = -1

Given m=1m = 1, we have:

1×m1=1 1 \times m_1 = -1

So, m1=1m_1 = -1.

Step 3: Use the point-slope form of a line yy1=m(xx1) y - y_1 = m(x - x_1) with point (3,15)(-3, -15) and slope 1-1:

y(15)=1(x(3)) y - (-15) = -1(x - (-3))

Simplify the equation:

y+15=1(x+3) y + 15 = -1(x + 3)

Expand:

y+15=x3 y + 15 = -x - 3

Subtract 15 from both sides:

y=x315 y = -x - 3 - 15

y=x18 y = -x - 18

Rearrange to the standard form:

y+x=18 y + x = -18

The equation of the line that passes through (3,15)(-3, -15) and is perpendicular to 3y=4xy3y = 4x - y is:

y+x=18 y + x = -18

3

Final Answer

y+x=18 y+x=-18

Key Points to Remember

Essential concepts to master this topic
  • Slope Rule: Perpendicular lines have slopes that multiply to -1
  • Technique: From y = x, perpendicular slope = -1 since 1 × (-1) = -1
  • Check: Substitute (-3, -15): -15 + (-3) = -18 ✓

Common Mistakes

Avoid these frequent errors
  • Forgetting to find negative reciprocal for perpendicular slope
    Don't use the same slope as the original line = parallel instead of perpendicular! This creates lines that never intersect rather than forming 90° angles. Always multiply slopes to get -1 for perpendicular lines.

Practice Quiz

Test your knowledge with interactive questions

Look at the linear function represented in the diagram.

When is the function positive?

–8–8–8–7–7–7–6–6–6–5–5–5–4–4–4–3–3–3–2–2–2–1–1–1111222333444555666777888–5–5–5–4–4–4–3–3–3–2–2–2–1–1–1111222333000

FAQ

Everything you need to know about this question

How do I know when lines are perpendicular versus parallel?

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Perpendicular lines: slopes multiply to -1 (like 2 and -1/2). Parallel lines: slopes are equal (like 3 and 3). Remember: perpendicular = negative reciprocal!

Why do I need to simplify the original equation first?

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You must find the actual slope of the given line! The equation 3y=4xy 3y = 4x - y looks complicated, but when simplified to y=x y = x , the slope is clearly 1.

What if the perpendicular slope comes out as a fraction?

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That's normal! If the original slope is 23 \frac{2}{3} , then the perpendicular slope is 32 -\frac{3}{2} . Just flip the fraction and change the sign.

Can I write the final answer in different forms?

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Yes! y+x=18 y + x = -18 is the same as y=x18 y = -x - 18 or x+y+18=0 x + y + 18 = 0 . Choose the form that matches your answer choices!

How do I check if my point actually lies on the line?

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Substitute your point coordinates into your equation. For (-3, -15): 15+(3)=18 -15 + (-3) = -18 ✓. If both sides equal, your line passes through the point!

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