My math textbook says that "y is directly proportional to x if there is a positive constant k such that y = kx"
But of course Wolfram Alpha says that "Two quantities y and x are said to be directly proportional, proportional, or "in direct proportion" if y is given by a constant multiple of x, i.e., y=cx for c a constant." where the condition on c being positive is not given.
But my teacher said two quantities are directly proportional if one quantity increases as the other increases or one quantity decreases the other quantity decreases i.e. implying k is positive, following the 1st definition. But I don't agree with this.