a=1

a=1
b=2

a=3;

a

a={3,2,1}

a

a[[3]]

f=x^2

x=2
f

g[y_]=y^2

g[3]

x=.

Plot[{g[x],Exp[x]},{x,0,2*Pi}]

eqn=D[y[t],t,t]+y[t]==0

soln=NDSolve[{eqn,y[0]==1,y'[0]==0},y[t],{t,0,2*Pi}]

ysol[t_]=y[t]/.soln[[1]][[1]]

Plot[ysol[t],{t,0,2*Pi}]

FindRoot[ysol[t]==0.3,{t,4}]

plot1=ParametricPlot[{Cos[t],Sin[t]},{t,0,2*Pi},PlotStyle->Blue]

plot2=ParametricPlot[{t*Cos[t],t*Sin[t]},{t,0,2*Pi},PlotStyle->Green]

Show[plot1,plot2,PlotRange->{{-3,7},{-5,2}}]