# Parallel tangent lines

## Homework Statement

Determine the equations of the tangent lines to the graph of f(x)=3x(5x^2+1) that are parallel to the line y=8x+9

y=m(x-x_1 )+y_1

## The Attempt at a Solution

11.f(x)=3x(5x^2+1)
The slope of the tangent line y=8x+9
f^' (x)=3(5x^2+1)+3x(10x)
f^' (x)=15x^2+30x^2+3
f^' (x)=45x^2+3
Finding the point of tangency:
m=8
8=45x^2+3
5=45x^2
√(x^2 )=√(45/5)
x=±1/3
f(1/3)=(3(1/3))(5(1/3)^2+1)
f(1/3)=1(5(1/9)+1)
f(1/3)=5/9+9/9=14/9
f(1/3)=14/9

f(-1/3)=(3(-1/3))(5(-1/3)^2+1)
f(-1/3)=-1(5(1/9)+9/9)
f(-1/3)=-1(5/9+9/9)
f(-1/3)=-14/9

y=m(x-x_1 )+y_1
y_1=14/9 x_1=1/3 m=8
y=8(x-1/3)+14/9
y=8x-8/3+14/9
y=8x-24/9+14/9
y=8x-10/9

y=m(x-x_1 )+y_1
y_1=-14/9 x_1=-1/3 m=8
y=8(x-(-1/3))-14/9
y=8x+8/3-14/9
y=8x+24/9-14/9
y=8x+10/9
Therefore, the equations of the tangent lines are y=8x+10/9 and y=8x-10/9

I'm pretty sure I did this correctly, but i'm not confident in my final statement.

Related Calculus and Beyond Homework Help News on Phys.org
lekh2003
Gold Member
You seem correct.

• Physics345
FactChecker
Gold Member
It looks good to me. After all those calculations, it's good to plug numbers back in and make sure it works as advertised: At x=1/3, does f' = 8 and f(x) = 3x(5x^2+1) = 8x+10/9? Do the same type of checks for x=-1/3.

• Physics345
It looks good to me. After all those calculations, it's good to plug numbers back in and make sure it works as advertised: At x=1/3, does f' = 8 and f(x) = 3x(5x^2+1) = 8x+10/9? Do the same type of checks for x=-1/3.
Of course, I did that on paper, but I thought it would be pointless, to show to the teacher when I handed in my work. Especially since my teacher understands my enjoyment and capabilities when it comes to math. Thanks guys, I appreciate your advice.

• lekh2003
FactChecker
Gold Member
You said that you were not confident in your answers. Plugging the numbers back in to verify that the answers are correct should have given you confidence.

The one exception is in the slope of 8, which depends on whether you calculated the derivative correctly. That is hard to verify by plugging numbers directly into f(x) except by plugging in x=1/3 and something like x+Δx = 1/3+0.001.

• Physics345
lekh2003
Gold Member
Plugging in numbers back again is a very fast way to figure out whether your answer is right, especially under time-intensive conditions or when the actual working is so long that you need an alternative method rather than skimming the working.

I just today used the technique of plugging the numbers back in to the start in a long time-intensive financial math test, where most of the working is hidden in the calculator.

You said that you were not confident in your answers. Plugging the numbers back in to verify that the answers are correct should have given you confidence.

The one exception is in the slope of 8, which depends on whether you calculated the derivative correctly. That is hard to verify by plugging numbers directly into f(x) except by plugging in x=1/3 and something like x+Δx = 1/3+0.001.
Oh I was referring to my therefore statement when I said that sir, and when I said "I'm pretty sure I did this correctly" I was referring to the math and using "pretty sure" was my scapegoat in case my math was somehow wrong in a way beyond, my spectrum of understanding. Sorry for the confusion! I have a problem conveying my thoughts through writing, it's something I've been working on a lot lately.

Last edited:
Plugging in numbers back again is a very fast way to figure out whether your answer is right, especially under time-intensive conditions or when the actual working is so long that you need an alternative method rather than skimming the working.

I just today used the technique of plugging the numbers back in to the start in a long time-intensive financial math test, where most of the working is hidden in the calculator.
Yes of course, I completely agree. The number one rule of doing math is checking your work, since there is always a way to do it in algebraically when it comes to functions, and by "always" I mean in relation to my current level of math.

FactChecker
Gold Member
Sorry for the confusion! I have a problem conveying my thoughts through writing, it's something I've been working on a lot lately.
Ha! I can really understand that! Being able to write clearly is something I have struggled with all my life. It's very good that you recognize how important that is.

• Physics345
Ha! I can really understand that! Being able to write clearly is something I have struggled with all my life. It's very good that you recognize how important that is.
Oh, I've always had this problem as well, unless I spend endless hours reviewing/refining what I write. I wish there was a easier solution, but I tend to rush into things without thinking, which I have found extremely hard to stop. Do you have any techniques or advice that could possibly help me out? I honestly thought I was crazy and this wasn't normal, I'm glad I found someone that can relate to my problem.

FactChecker
• 