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-   -   Please help me- gradient of tangent (http://mymathforum.com/calculus/342585-please-help-me-gradient-tangent.html)

ashleybouchakian October 31st, 2017 07:57 PM

Please help me- gradient of tangent
 
f(x)=ax^2+c
dy/dx=2ax

Point A (-2,5) lies on the graph of y=f(x). the gradient of the tangent to this graph at A is -6. Find the value of a and c.

greg1313 October 31st, 2017 08:09 PM

Where are you having difficulty? The problem can be solved by straightforward substitution.

$$f'(-2)=2a(-2) = -6$$

Can you continue?

ashleybouchakian October 31st, 2017 08:11 PM

can you please finish it? I am not sure why, but I am so confused on finding both a and c. I have been doing math all day

Country Boy November 2nd, 2017 04:41 AM

Quote:

Originally Posted by ashleybouchakian (Post 583346)
f(x)=ax^2+c
dy/dx=2ax

Point A (-2,5) lies on the graph of y=f(x). the gradient of the tangent to this graph at A is -6. Find the value of a and c.

Since "(-2, 5) lies on the graph", f(-2)= a(4)+ c= 5. The "gradient of the tangent" is 2a= -6. Find a then put that value into the first equation and find c.


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