Keith Dysart wrote:
Ptotal = P1 + P2 + 2*SQRT(P1*P2)cos(A)

Curiosity question: Which of the two possible values for
the square root did you use? Elaborate on the reasons
for your choice?

Probably should be written 2*|SQRT(P1*P2)|*cos(A) to
satisfy the purists. The sign of the interference
term must match the type of interference which is
determined by the sign of cos(A).

The third term in the equation is the interference term.
A positive value indicates constructive interference.
A negative value indicates destructive interference.
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73, Cecil http://www.w5dxp.com