use of SetDelayed functions in Plot and Solve

*To*: mathgroup at smc.vnet.net*Subject*: [mg44585] use of SetDelayed functions in Plot and Solve*From*: "Mukhtar Bekkali" <mbekkali at hotmail.com>*Date*: Sat, 15 Nov 2003 02:05:13 -0500 (EST)*Organization*: Iowa State University*Sender*: owner-wri-mathgroup at wolfram.com

Why can't I plot the function and find the solution to the equation below using SetDelayed functions? Below is just an example, the real functions are symbolically non-integrable. s[t_] :=NIntegrate[t^2 - z*Log[t] , {z, 0, k[t]}] i[t_] := D[s[t],t] /. {k[t] ->2t,k'[t]->Sin[t]} Plot[i[t], {t, 1, 10}] Solve[i[t]==300, t]] All I get is an empty graph because I have "not machine real size numbers" and my equation "appears to be solved essentially in non-algebraic way". If I rewrite the code using simple Set I get my graph and solutions. s=Integrate[t^2 - z*Log[t] , {z, 0, k[t]}] i=D[s,t] /. {k[t]->2t,k'[t]->Sin[t]} Plot[i, {t, 1, 10}] Solve[i==300, t]] Very confused. Thanks, Mukhtar Bekkali

**Follow-Ups**:**Re: use of SetDelayed functions in Plot and Solve***From:*Anton Antonov <antonov@wolfram.com>