with(Student:-Calculus1): with(Maplets): with(Elements):

RiemannSumMaplet := 
 Maplet(
  Window['W1'](
    [ 
     ["Enter a function", 
        TextBox['myF']( value = "cos(x)", width = 6),

      "a = ", 
        TextBox['a']( value = "0", width = 3),

      "b = ", 
        TextBox['b']( value = "4", width = 3)],

     ["Number of partitions", 
        Slider['S1']( 0..100, 20, 'majorticks'=20, 'minorticks'=5, 'snapticks'='false')],

     [Plotter['P']( height = 400, width = 400)],

     [MathMLViewer['Sum']( height = 75, width = 250 ),
      DropDownBox['C']([ blue, green, black, brown, turquoise ])],

     [Button['B1']( "Show Riemann Sum", onclick = 'ShowTheSum')]
    ]
  ),

  Action['ShowTheSum'](
    Evaluate('Sum' = 'MathML:-Export(ApproximateInt( myF, x=a..b, method=left, 
                                                     output=sum, partition=S1))'),
    Evaluate('P' = 
       'ApproximateInt( myF, x=a..b, method=left, 
                        output=plot, boxoptions=[color=C], partition=S1, title="")')
  )
):

Display( RiemannSumMaplet );