ax=0.;bx=1;h = 0.1;nx = ((bx-ax)/h);
mrezhax=Table[ax+i*h,{i,0,nx}];
psi[x_]:=Sin[Pi*x]
psi[mrezhax]

за ((Subscript[U, 1])^1)

0.309+(1/2)(0.588-2*0.309)

uexact[x_,t_]:=Sin[Pi*x]Exp[-(Pi^2)*t]
uexact[0.1,0.005]