Equation for Line Of Best Fit?

I have a chart out of which I made a graph, On Microsoft Excel (2011 for mac). I then added a Line of Best Fit.

I need to answer questions about the graph, which include knowing the line of best fit. (Teacher said to use any statistical program to figure it out.....)

How do I get the equation for Line of Best Fit/what is the equation?

Re: Equation for Line Of Best Fit?

If you are trying to find an equation for y as a function of x in the form y= mx+c given you have a sample size n for observations of x and y with means X and Y

Find the sum of squares for y (SS_{yy})

$\displaystyle SS_{yy}= \sum_{i=1}^n (y_iy_i)-nY^2$

And the sum of squares for xy

$\displaystyle SS_{xy}= \sum_{i=1}^n (x_iy_i)-nXY$

Then

$\displaystyle m=\frac{SS_{xy}}{SS_{yy}}$

$\displaystyle c= Y-m X$

Re: Equation for Line Of Best Fit?

Appreciate your reply, but it looks like chinese to me :(.

This is the "Data" I have been given:

**Annual Income Of Canadian Earners**

Year/ Lowest 20% Of Earners /Highest 20% Of Earners

1977 12000 122500

1978 11800 126400

1979 13100 121100

1980 13700 123200

1981 14600 123200

1982 14300 122300

1983 12800 127600

1984 13900 126300

1985 14800 130700

1986 15100 134300

1987 15800 137000

1988 15700 142800

1989 16800 147800

1990 15500 143100

1991 14300 139100

1992 13900 137400

1993 14100 136700

1994 14100 136300

1995 14300 139900

1996 13000 141900

1997 13200 146500

1998 13600 153900

1999 13500 162300

2000 14300 169600

2001 14700 169400

2002 14700 167800

2003 14800 166700

2004 14200 170600

2005 14300 168500

2006 14700 168000

Re: Equation for Line Of Best Fit?

You say you put this into Excel and "added a Line of Best Fit". I interpret that to mean that you used an Excel command to plot that line. Does Excel not have a way of giving the equation? If not you could still find two points on that line and use those to find the equation of the line. Shakarri gives the standard formulas for the coefficients of the line but Excel should already have done that for you.

Re: Equation for Line Of Best Fit?

if you go to your data points that excel plotted, select one and right click then go to "Add trendline" and at the bottom of the window that pops up is a box that says "Display equation on chart" select it and voila!

Re: Equation for Line Of Best Fit?

^Thanks! Thought there HAD TO be a function in excel that did it! :)