Trendline Linear Algorithm
 

The trendlines in Excel use regression analysis. For a straight line analysis, go get a college math text and look up the section on "Linear Regression". You are looking for an equation of the form y=a+bx. Regression analysis of "n" data points will give you the coefficient b={SUM(x*y) - 1/n*[SUM(x)*SUM(y)]}/{SUM(y^2)-1/n*SUM(y)^2}. Then you plug in "b" using one of your ordered pairs and solve for "a". An excellent reference is "The VNR Concise Encyclopedia of Mathematics" by W. Gellert, H. Kustner, M. Hellwich and H. Kastner, published by Van Nostrand Reinhold Co. ISBN 0-442-22646-2. page 599. Also, most editions of the "CRC Standard Math Tables and Formulae" have the information.