What am I dong wrong here?

  • #1

Homework Statement



If y' = 6x(x2+2)2 and y = 25 when x = 1, what is the value of y when x = -1?

Homework Equations

N/A

3. The Attempt at a Solution [/B]

∫6x(x2+2)2dx = 2x(x2+2)3

(12 + 3)3 + 2(1) + c = 25

1 + 2 = 3 and 2(1) = 2 and 33 = 27, so 27*2 + c = 25 so 54 + c = 25, therefore c = 25-54 = -29

Now substitute x = -1

(-1) squared is 1 and 2(-1) = -2, so 27*-2 = -54, so -54 - 29 = y = -83

WileyPlus tells me it's wrong.

What is wrong with this?
 

Answers and Replies

  • #2
SammyS
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Homework Statement



If y' = 6x(x2+2)2 and y = 25 when x = 1, what is the value of y when x = -1?

Homework Equations

N/A

3. The Attempt at a Solution [/B]

∫6x(x2+2)2dx = 2x(x2+2)3
You need a constant of integration in that result.

Also, you made a mistake in your integration.

What is the derivative of y = 2x(x2+2)3 + C ?

(12 + 3)3 + 2(1) + c = 25

1 + 2 = 3 and 2(1) = 2 and 33 = 27, so 27*2 + c = 25 so 54 + c = 25, therefore c = 25-54 = -29

Now substitute x = -1

(-1) squared is 1 and 2(-1) = -2, so 27*-2 = -54, so -54 - 29 = y = -83

WileyPlus tells me it's wrong.

What is wrong with this?
 
  • #3
You need a constant of integration in that result.

Also, you made a mistake in your integration.

What is the derivative of y = 2x(x2+2)3 + C ?
OH! *smacks self* Never mind...

And I just forgot to type in the + c; you'll see in my calculations that it appears there, sorry.

EDIT: Thanks, BTW, like a LOT!
 
  • #4
Thanks a lot, SammyS!
 

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