Lines can be used to approximate a wide variety of functions; often a function can be described using many lines. if a stock price goes from $10 to $12 from january 1st to january 31, and from $12 to $9 from february 1st to february 28th, is the price change from $10 to $9 a straight line? how can i use two “pieces” of lines to describe the price movements from the beginning of january to the end of february?