Posted by Ronnie on Saturday, June 19, 2010 at 1:06pm.
I have been struggling with this problem I cannot get the right answer. Please give me the steps to solve this problem.
Let f(x)= (2x9)(x^2 + 6)
(A) Find the equation of the line tangent to the graph of f(x) at x=3
(B) Find the values(s) of x where the tangent line is horizontal.

College Math  Reiny, Saturday, June 19, 2010 at 5:11pm
I would expand the function first, then find the derivative.
f(x) = 2x^3 + 12x  9x^2  54
f '(x) = 6x^2 + 12  18x
when x = 3
f(3) = 3(15) = 45
f '(3) = 6(9) + 12  18(3) = 12
A) so you have a slope of 12 and a point (3,45)
Use y = mx + b to find the tangent equation
B) set 6x^2  18x + 12 = 0
and solve
Hint: divide both sides by 6, then it factors

College Math  Anonymous, Friday, June 25, 2010 at 4:58pm
Identify the variable, constant, and coefficient of the expression: –10k + 15
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