# Thread: Finding the differential of dy

1. ## Finding the differential of dy

Let

Find the differential when and

Find the differential when and

so far i came up with dy/dx = 10x+7

dy = [10x+7]dx

so now i am confused and i don't know if i did that right and i need to know what to do next

2. I believe you should have put this in the differential equation section of the forum, not calculus.
PM a moderator and get them to move your post.

3. Originally Posted by jpratt

so far i came up with dy/dx = 10x+7

dy = [10x+7]dx

so now i am confused and i don't know if i did that right and i need to know what to do next
Substitute some values in?

4. Originally Posted by Jskid
I believe you should have put this in the differential equation section of the forum, not calculus.
PM a moderator and get them to move your post.
No, just "finding differentials" has nothing to do with differential equations.

5. Originally Posted by jpratt
Let

Find the differential when and

Find the differential when and

so far i came up with dy/dx = 10x+7

dy = [10x+7]dx

so now i am confused and i don't know if i did that right and i need to know what to do next
Yes, that is the general formula for dy. Now just do what they say: set x= 1, dx= .03 and do the arithmetic.

6. In my opinion the terms dx and dy don't indicate differentials but finite differences [otherwise an expression like dx= .3 is meaningless...]. In that case writing in the equation...

$\displaystyle y = 5\ x^{2} + 7\ x + 4$ (1)

... y + dy instead of y and x + dx instead of x You obtain...

$\displaystyle y + dy = 5\ (x + dx)^{2} + 7\ (x+dx) + 4 \rightarrow dy = 10\ x\ dx + 5\ dx^{2} + 7\ dx$ (2)

... that allows You to find the solution...

Kind regards

$\displaystyle \chi$ $\displaystyle \sigma$