f[x_]:=Sin[x];
x1=0;
x2=1;

Manipulate[Plot[{f[x],f'[k]*x+f[k]-f'[k]*k,(f[x1]-f[x2])/(x1-x2)*x+(x1*f[x2]-x2*f[x1])/(x1-x2)},{x,x1,x2},AxesLabel->{"x","y"}],{k,x1,x2}]