How do you use the definition of a derivative to find the derivative of #f(x)=-3x^2+2x+9#?

1 Answer
Feb 28, 2017

Answer:

#(df_x)/(dx)=color(green)(-6x+2#
(see below for method using the definition of derivative)

Explanation:

The derivative of a function #f(x)# (with respect to #x#) is defined as
#color(white)("XXX")(df_x)/(dx)=lim_(hrarr0) (f(x+h)-f(x))/h#

Given: #f(x)=-3x^2+2x+9#

#color(white)("XXX")f(x+h)=-3(x+h)^2+2(x+h)+9#
#color(white)("XXXXXXXX")=-3x^2-3xh-3h^2+2x+2h+9#

#{: (f(x+h)," = ",,-3x^2,-6xh,-3h^2,+2x,+2h,+9,), (-f(x)," = " ,"- [ ",underline(-3x^2),underline(color(white)(_3xh)),underline(color(white)(-3h^2)),underline(+2x),underline(color(white)(+2h)),underline(+9)," ]"), (f(x+h)-f(x)," = ",,,-6xh,-3h^2,,+2h,,) :}#

#rArr color(white)("XX")(f(x+h)-f(x))/h=-6x-3h+2#
and
#color(white)("XXX")lim_(hrarr0) (f(x+h)-f(x))/h=-6x-3 * (0) + 2 = color(green)(-6x+2)#