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I found one difficult equation for me...

It is:

Valid for real numbers a, b, c, d:

a + b = c + d

ad = bc

ac + bd = 1

What is the maximum of a+b+c+d?

And DON'T SAY wolfram, really no wolfram... I used it and it isn't good.

So... have you got any idea?

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# Equations -maximum

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