I am a new learner of Mathematica, I think this question should be simple, but I just can’t get the correct answer…
This is my equations:
p=x+y q=2x+y r=p+q
And I want to get the derivation of r by x, so the answer should be 3 (r=(x+y)+(2x+y)=3x+y)
But the output I got is a+b ??!! Why?