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Hello, Fnus!
We are told: .
Let be a point between the two sources.
. . Let = distance from "40".
. . Then = distance from "5".
The total intensity is: .
Then we have: .
Multiply by
This simplifies to: .
. . which factors: .
. . and has the real root: .
Since is always positive,
. . the function is always concave up.
Therefore, provides minimum