Calculus Calculus Math Forum

 October 31st, 2017, 06:57 PM #1 Newbie   Joined: Oct 2017 From: California Posts: 2 Thanks: 0 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.
 October 31st, 2017, 07:09 PM #2 Global Moderator     Joined: Oct 2008 From: London, Ontario, Canada - The Forest City Posts: 7,923 Thanks: 1123 Math Focus: Elementary mathematics and beyond Where are you having difficulty? The problem can be solved by straightforward substitution. $$f'(-2)=2a(-2) = -6$$ Can you continue? Thanks from ashleybouchakian
 October 31st, 2017, 07:11 PM #3 Newbie   Joined: Oct 2017 From: California Posts: 2 Thanks: 0 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
November 2nd, 2017, 03:41 AM   #4
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Quote:
 Originally Posted by ashleybouchakian 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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