\begin{pspicture}(-0.3,-0.4)(6.6,2.8) %\psgrid(0,0)(6.6,2.8) \psline[linecolor=blue, linewidth=0.5pt]{->}(0,0)(4.7,0) \psline[linecolor=blue, linewidth=0.5pt]{->}(0,0)(0.7,2) \psline[linecolor=blue, linewidth=0.5pt](0.7,2)(5.4,2) \psline[linecolor=blue, linewidth=0.5pt](4.7,0)(5.4,2) %, linestyle=dashed \psline[linecolor=red, linewidth=0.5pt, linestyle=dashed]{->}(0,0)(1,0.6) \psline[linecolor=red, linewidth=0.5pt, linestyle=dashed](1,0.6)(5.7,0.6) \psline[linecolor=red, linewidth=0.5pt](4.7,0)(5.7,0.6) \psline[linecolor=red, linewidth=0.5pt](5.4,2)(6.4,2.6) \psline[linecolor=red, linewidth=0.5pt](5.7,0.6)(6.4,2.6) \psline[linecolor=red, linewidth=0.5pt](0.7,2)(1.7,2.6) \psline[linecolor=red, linewidth=0.5pt ,linestyle=dashed](1,0.6)(1.7,2.6) \psline[linecolor=red, linewidth=0.5pt](1.7,2.6)(6.4,2.6) %\rput[c](0.25,1.15){\color{blue}\tiny{+}} \rput[c](2.5,-0.25){$\vec{a}$} \rput[c](0.15,1.1){$\vec{c}$} \rput[c](0.5,0.6){$\vec{b}$} \end{pspicture}