In this equation '' f (x) = x^4 + 3x -9'', why are 3, 9 constants and x variable if I do the differentiation and the integration?
f (x) = x^4 + 3x -9
f ' (x) = 4x^3 + 3
∫ x^4 + 3x - 9 dx = x^5 / 5 + 3x^2 / 2 - 9x
+55 
