I am studying Control Theory from Ogata. My math is a little rusty, so this is a math question about Frequency Domain Analysis. Check this page - http://www.flickr.com/photos/66943862@N06/6337116280/sizes/l/in/photostream/ [Broken] I get everything upto Equation 8-4 However I don't get the line after that. The line "where the constant a can be evaluated from Equation (8-2) as follows" And they write the value of a. How is a = G(s) (ωX) /(s^2 + ω^2). How do they arrive at this value of a? Can someone help? And why did they substitute s = -jω after that?