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It is not as tough as you think. Take a deep breath, clear your mind then read very carefully the following that I repeat from post #24.this is a tough one...still struggling, my latest attempt

## -(1/a +1/b+1/c+1/d)= abcd##..........attempt 1

and

##-(cd(a+b)+ab(c+d)=4##

##cd(-1-c-d)##+##\frac 2 {cd} (c+d)=4##........attempt 2, am i on the right path?

Can you achieve this goal by looking at equations that do not contain ##A##?Your goal is to find an expression for ##A##, meaning that you need to have ##A## alone on the left hand side and some expression on the right hand side involving ##a##, ##b##, ##c## and ##d##.