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My tutor gave me a hint which confuses me.

A=-4 integral y dx where the upper limit is pi/2 and the lower limit is 0.

I do not understand the -4 part. why must we multiply the integral by -4?

2) Find the volume of the solid generated by revolving about the y-axis the area between the first arc of the cycloid x=t-sin(t), y=1-cos(t) and the x-axis. Use the shell method.

v=2pi integral xy dx

=2pi integral (t-sin(t)) (1-cos(t))^2 dt

I'm stuck at this point.