I am doing a sketching a curve problem and one of the guidelines in my book is " Intervals of increase or Decrease" which says to use the I/D Test. The original is f(x) = x^3 + x. can anyone help?

Question

anonymous · Answer

what is the "id" test?
if it is calculus you can take the derivative, set it equal to zero and find the critical points
but the derivative is $3x^2+1$which is always positive, so this function is always increasing