>

restart:

>	AreaApprox:=(5*Pi/2000)*sum(2+sin(-3*Pi/2+5*Pi*i/2000),i=1..1000):

>	evalf(AreaApprox);

You need to enter Pi rather than pi to get the decimal approximation.

>

i:=4;

>	AreaApprox:=(5*Pi/2000)*sum(2+sin(-3*Pi/2+5*Pi*i/2000),i=1..1000):

Error, (in sum) summation variable previously assigned, second argument evaluates to 4 = 1 .. 1000

If "i" already has a value assigned then you have a problem.  The following will solve the problem so that you do not have to worry about any value being previously assigned to i.

>	AreaApprox:=(5*Pi/2000)*sum('2+sin(-3*Pi/2+5*Pi*i/2000)','i'=1..1000):

>	evalf(AreaApprox);

The quotes ' ' prevent Maple from evaluating the i as 4 while computing the sum.