rectXCoord= {0,0.6}, {0.6,1}, {1,2}, {2,2.4}, {2.4,3};bars=Map[Graphics[ EdgeForm[{Thickness[Medium],Black}], Red, Polygon[{{#[[1]],0},{#[[2]],0},{#[[2]],Sin[#[[1]]]},{#[[1]],Sin[#[[1]]]}}]]&,rectXCoord];rectArea=Plus@@ Map[ (#[[2]]-#[[1]])*Sin[#[[1]]]&, rectXCoord];Grid[ {Show[Flatten[{functionPlot,bars,functionPlot}],ImageSizeMedium]}, {"Rectangle area "<>ToString[rectArea//N]}, {"Actual area "<>ToString[area//N]}, {"Error "<>ToString[error//N]}]