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We know that:
Hx = X est + delta 
( x )
Hy = Y est + delta 
( y )
Also
Est Di = (Lhi - Xest) + (Hy - Yest) .......................(4)
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d(EstDi) EstDi . delta ( x ) d(EstDi) . delta( y )
PDi = Est Di + -------------- + -----------------
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