Tax table: Here are selected entries from the 2007 tax table that show the federal income tax owed by those married and filing jointly. The taxable income and the tax are both in dollars. Over what...

Tax table: Here are selected entries from the 2007 tax table that show the federal income tax owed by those married and filing jointly. The taxable income and the tax are both in dollars.

Over what two parts of this table is the tax a linear function of the taxable income? Find formulas for both linear functions, and explain in practical terms what the slopes mean.