It seems only a math problem, but I can't understand it.

[zhulaojianke]

Just Like the pictures and the worksheet bellow, I have only changed the y's assignment position, but the result is different! So I hope someone could help me. Thanks a lot.

[AndyWesterman · Accepted solution]

It is a problem that needs differentiation by parts

Your function is d/dx of a(x)*b(x) so the result should be: (d/dx a(x) ) *b(x) + a(x) * (d/dx b(x))

Unfortunately if you define f(x,y) mathcad is assuming that these parameters are independant

When later you let mathcad know that y=x^5 then it substitutes that into the result it already had,

but information is lost & you get an incorrect answer.

[zhulaojianke]

……but information is lost ……

OK, I should tell mathcad the informations in order. Really! I've forgotten the difference of relationship between x and y in different assignment position. Thank you very much!