TITLE: Differentiating the polynomial $x^3 - 4x +6$
QUESTION [5 upvotes]: I am suppose to differentiate $$x^3 -4x +6,$$ but I can not get the correct answer. I get $\frac{\mathrm{d} }{\mathrm{d} x}(x^3) = 3x^2$ using the power rule and then $\frac{\mathrm{d} }{\mathrm{d} x}(4x) = 4$ and $\frac{\mathrm{d} }{\mathrm{d} x}(6) = 1$. This is not correct but I do not know why.

REPLY [2 votes]: Use the rule of the sum of functions, $$\frac d{dx}(x^3-4x+6)=\frac d{dx}(x^4)-\frac d{dx}(4x)+\frac d{dx}(6)=\underline{3x^2-4}\\\text{NOT }\;\; 3x^2-4 \;\;\underline{+ 6} $$