Finding the derivitive by definition

[j814b815]

Hello and thanks for viewing

I have just started with derivatives and I am having difficulties with the following problem:

Find the equation of the tangent line to the curve at the given point.

y = 4x - 3x² , (2, -4)

I am using the formula f(x + h) - f(h) / h

When I use this formula I get my X's to cancel and end up with -3h² + 4h / h

Divide by h and get -3h + 4

This differs from the answer in the book. Where did i go wrong? ( I know more steps are involved in the problem but it seems the differentiation is where I go wrong)

 

[Deleted member 4993]

Hello and thanks for viewing

I have just started with derivatives and I am having difficulties with the following problem:

Find the equation of the tangent line to the curve at the given point.

y = 4x - 3x² , (2, -4)

I am using the formula [f(x + h) - f(h)] / h

When I use this formula I get my X's to cancel and end up with [-3h² + 4h] / h

Divide by h and get -3h + 4

This differs from the answer in the book. Where did i go wrong? ( I know more steps are involved in the problem but it seems the differentiation is where I go wrong)

First of all you need to use grouping symbols to control order of operation.

What expression did you get for f(x+h)?

 

[HallsofIvy]

Hello and thanks for viewing

I have just started with derivatives and I am having difficulties with the following problem:

Find the equation of the tangent line to the curve at the given point.

y = 4x - 3x² , (2, -4)

I am using the formula f(x + h) - f(h) / h

When I use this formula I get my X's to cancel and end up with -3h² + 4h / h

Then try it again. f(x+h)- f(h)= 4h- 6xh+3h^2. Surely, you didn't think (x+h)^2= x^2+ h^2???

Divide by h and get -3h + 4

This differs from the answer in the book. Where did i go wrong? ( I know more steps are involved in the problem but it seems the differentiation is where I go wrong)