Find f(a+h)-f(a) for the function

Question

f(x)= x² -15x +9

Answer 1

f(a+h) - f(a) = (a +h)² - 15(a + h) + 9 - (a² - 15a + 9)

 

                        = a² + 2ah + h² - 15a - 15h + 9 - a² + 15a - 9

 

                        = 2ah + h² - 15h

Answer 2

f(x) = x2 - 15x + 9

 

f(a+h) = (a+h)² - 15(a+h) + 9 = a² + 2ah + h² - 15a - 15h + 9

 

f(a) = a² - 15a + 9

 

f(a+h) - f(a) = h² + 2ah - 15h