restart;
with(Student[Calculus1]);

ApproximateInt(sqrt(1-x^2), x = -1/2 .. 1/2, 
    method = trapezoid, partition = 36, output = plot);

T := ApproximateInt(sqrt(1-x^2), x = -.5 .. .5, 
    method = trapezoid, partition = 36);

A := int(sqrt(1-x^2), x = -1/2 .. 1/2);

evalf(abs(T-A));

10.0^(-4);