Plot[Sin[x]/x, {x, -10, 10}]

Sinc[x_] := If[x == 0, 1, Sin[x]/x]

Sinc[0]

Plot[{Sin[x]/x, 1, 1 - Abs[x]}, {x, -10, 10}, 
      PlotStyle -> {Red, Blue, Green}]

Plot[{Log[1 + x]/x, 1, (1/(1 + x))}, {x, -1, 2}, 
      PlotStyle -> {Red, Blue, Green}, 
      PlotRange -> {0, 2}]