I should have said a couple more things. First, when integrand
evaluates to something other than the zero function (or, at least, to
a non-constant function), the original code
var('x,y,t')
F=vector([x^2,x*y])
r=vector([cos(t), sin(t)])
tstart=0
tend=2*pi
integrand = F(x=r[0], y=r[1])*diff(r,t)
seems to work just fine. Also, I am not running Sage locally, but
rather through a browser at sagenb.org.
Thomas L. Scofield
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Associate Professor
Department of Mathematics and Statistics
Calvin College
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