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October 31st, 2017, 07:57 PM   #1
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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.
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October 31st, 2017, 08:09 PM   #2
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Where are you having difficulty? The problem can be solved by straightforward substitution.

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

Can you continue?
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October 31st, 2017, 08:11 PM   #3
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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
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November 2nd, 2017, 04:41 AM   #4
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Quote:
Originally Posted by ashleybouchakian View Post
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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