algebra/precalc  please help
posted by melissa .
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?
I don't believe it will be a straight line, but how do I do the second part of using two pieces of lines to describe the price movements?

If we use x as the number of days since the start of the year,
build each piece with a y=mx+b section, where the price starts a t b and the line has slope m ($/day)
January is y = 10 + 2/31 x
February is 12  3/28 (x31)
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