### Answer:

16

### How:

a^2+b^2=c^2+d^2=2

From this we can say, a=b=c=d=±1

So, ac+bd = (±1)(±1)+(±1)(±1) = 1+1 = 2

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if a,b,c,d are positive real numbers and a^2+b^2=c^2+d^2=2 then what is the max value of ac+bd?

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