What is the difference between z[x_]:= D[y[x],x] and z[x_]:=y'[x]?

• To: mathgroup at smc.vnet.net
• Subject: [mg27436] What is the difference between z[x_]:= D[y[x],x] and z[x_]:=y'[x]?
• From: liwen liwen <gzgear at yahoo.com>
• Date: Sun, 25 Feb 2001 20:55:39 -0500 (EST)
• Sender: owner-wri-mathgroup at wolfram.com

```Dear friends,
How are you!
I want to know the difference between z[x_]:=
D[y[x],x] and z[x_]:=y'[x].

Also,I want to why I can not find the answer for the
u[0.5] by the following routine:

(r1[t_] = 93/(1 + 0.12*Cos[3*t]);
d1[t_] := ArcTan[r1[t]/D[r1[t], t]];
Block[{t}, u[t_] := If[(Pi/3 >= t > 0), d1[t]]];
)

u[0.5]
General::"ivar": "\!\(0.5`\) is not a valid variable."

But it is available by the following routine:

(r1[t_] = 93/(1 + 0.12*Cos[3*t]);
d1[t_] = ArcTan[r1[t]/D[r1[t], t]];
Block[{t}, u[t_] := If[(Pi/3 >= t > 0), d1[t]]];
)

u[0.5]
1.22872
(* ------------------------ *)

Bets Regards,

Liwen  2/26/2001

E-mail: gzgear at yahoo.com

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```

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