Comment by litoE

Back in 1969 I had a summer job in the engineering department of a factory. The Chief Engineer was an old guy who was a slide rule artist. He taught me how to solve second degree equations (ax^2 + bx + c = 0) with just a slide rule. The standard formula that I had learned in high school includes adding and subtracting, which a slide rule cannot do. His method involved pulling out the slider of the slide rule and inserting it backwards. Then, through some clever manipulation using the D and B scales (which now faced each other) and the A and C scales, he could actually solve the equation with just the slide rule. I wish I could remember exactly how it was done.