kill(all)$

van=facebook.com/LearningCalculus$

integrate(abs(sin(3*x)+sin(2*x) + sin(x)),x,0,%pi) =
    1*'integrate(sin(1*x),x,0,%pi) +
    2*'integrate(sin(2*x),x,0,%pi) +
    3*'integrate(sin(3*x),x,0,%pi);

term1: 1*integrate(sin(x),x,0,%pi);

term2: 2*integrate(sin(2*x),x,0,%pi/2);

term3: 3*integrate(sin(3*x),x,0,%pi/3);

term1 + term2 + term3;

float(%);