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# Incorrect substitution into fraction

Posted 1 year ago
 I input (x + y)/(z + w), x = A*y, z=B*w  to WolframAlpha, yielding yet by substitution, we find $$\frac{x + y}{z + w} = \frac{Ay + y}{Bw + w}$$ Is there a way to query that forces correct output? This is a minimal reproduction of a more complicated original where I can't just plug in.
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Posted 1 year ago
 Uh oh... I trusted several outputs with my input format, where I couldn't tell it was mistaken. I think this is a seriously misleading and should be addressed.Replacing the fraction with x / y yields 0, which makes no sense, yet w / z works as expected. In the original fraction, WA arbitrarily setting x=0 instead of just leaving it as x + y also makes no sense.You appear to be correct, thanks! where x = A*y, z = B*w also works.(Let me know if I can "accept" a post like on Github or Stack Exchange, as the resolution)
Posted 1 year ago
 I don't use WolframAlpha much, so I'm inferring a lot here. The {} you see in WA's interpretation of your input makes me think it naturally assumes comma separated expressions should be interpreted as a list. And the fact that only one substitution was applied in the next block makes me think that WA assumes that substitutions are defined by the last expression given.I was able to get it to do what I think you want with this input (x + y)/(z + w) where x = A*y and z = B*w 
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