For me, this is a favorite equation only because its been pivotal for my research, a popular equation found in biostatistics, its called
Linear extrapolation :
Y (Xn) = Yk-1 + ( Xn-Xk-1/ Xk-Xk-1) (Yk-Yk-1)
Linear extrapolation means creating a tangent line at the end of the known data and extending it beyond that limit. Linear extrapolation will only provide good results when used to extend the graph of an approximately linear function or not too far beyond the known data; when involving multiple factors, one can go into polynomial extrapolation and my other favorite (somewhat) conic extrapolation.
Who knew math would be so necessary in bioinformatics and medical science? lol
Everything is inter-related. 
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