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Exp Rel Function
Math Engineering Class

Public Function ExpRel( _
    ByVal vX As Variant _
    ) As Variant

Calculate the relative error exponential (EXP(X)-1)/X.

Examples: 
    ExpRel(-5.0) = 0.198652410600183
    ExpRel(-1.0) = 0.632120558828558
    ExpRel(-0.5) = 0.786938680574733
    ExpRel(+0.0) = 1
    ExpRel(+0.5) = 1.29744254140026
    ExpRel(+1.0) = 1.71828182845905
    ExpRel(+5.0) = 29.4826318205153
See also: 
    LnRel Function
    DEXPRL Function
vX: Function returns Null if vX is Null or cannot be fixed up to a Double precision floating point number.

Evaluate 

    EXPREL(X) = (EXP(X) - 1.0) / X
For small ABS(X) the Taylor series is used. If X is negative the reflection formula 
    EXPREL(X) = EXP(X) * EXPREL(ABS(X))
may be used. This reflection formula will be of use when the evaluation for small ABS(X) is done by Chebyshev series rather than Taylor series.

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