# Spivak Chapter 4 Question 7

In particular, I'm not sure what a straight line is defined as in Spivak. Earlier in the text, Spivak defines a straight line as a certain collection of pairs, including, among others, the collections {$$(x, cx)$$: x a real number}. However, it doesn't seem right to use this definition of a straight line - that straight line is a set of all points such that {$$(x, -(A/B)x + C)$$: x a real number}, as it would make the problem trivial. So what should I treat the definition of a straight line as, for this problem?
• That seems okay. Alternatively, a line is a curve whose second derivative is $0$. May 6 at 12:35
A straight line, if it's not vertical can be written in the form of $$(x, mx+c)$$. If it's vertical, then it can be written as $$(x_0, y)$$ where $$x_0$$ is fixed.