1+2

23^4*2+4/2

eq:= x+y

eq /. {x->1,y->2}

f[x_,y_]:=x+y

f[1,2]

Solve[x^2-2==0,x]

N[Cos[1],50]

FindRoot[x-Sin[x]-1==0,{x,0,2}]