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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? 
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 
Math Team Joined: Jan 2015 From: Alabama Posts: 3,264 Thanks: 902  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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