Calculating Derivative of f[x] - Why Does Mathematica Give Incorrect Output?

[ktpr2]

I have

f[x_] := ax+b/2x+d
f'[x]

and I get

-2(ax+b)/(d+2x)^2

instead of

ad-2b/(d+2x)^2

Assuming the above, my answer, is correct, why does mathemetica give me its answer? Is it interpreting one of the variables as some kind of weird constant?

 

[ktpr2]

no one has any idea what might be wrong?

 

[chroot]

You're not using parentheses properly. The / operator has a higher precedence than the + operator. Try:

f[x_] := (ax+b)/(2x+d)

- Warren

 

[chroot]

Oops, nevermind. The problem is that Mathematica thinks "ax," without spaces, is a variable unto itself, different from either a or x. You need to use either spaces or the * operator to indicate multiplication:

f[x_] := (a*x+b)/(2x+d)

- Warren

 

[Delta]

Which answer is yours and which answer is mathematica's.

The answer I get is the ad-2b/(d+2x)^2, the -2(ax+b)/(d+2x)^2 comes from doing only half of the quotient rule equation.

EDIT: Or just follow the advice above :tongue:

 

[ktpr2]

that makes sense. thanks a lot. I'm seeing how delicate things have to be formated. For instance, I just ran into another thing where I realized that my trig functions have to have bracketed variable inputs (Cos [x]) as opposed to Cos (x) which is something else.