@Carl Love 

Oh, I see how to use the product() in this case. 

The hint for the question is to use the mul() but I can not find the document in the maple. 

And yes for n great that 0, the answer will be pi/2 and using for loop can prove that. 

Thank you so much

 @nm 

Thanks for your reminder. I will try my best to solve my problem. Thank you. 
@mmcdara 

How I can the real number rather than RootOf? Thank you!@Carl Love 

@tomleslie 

But I got this error when I run the code 

Error, invalid input: rhs received [{x = 0, y = -1}, {x = 0, y = 3}, {x = -2/3, y = -1}, {x = -2/3, y = 3}][j], which is not valid for its 1st argument, expr
 

Could you please help to sort it out? Thank you! 

Appreciate you have helped me several times! 

@acer 

Thank you so much!!!! 

@acer 

@tomleslie 

@nm Great! Thank you.

@acer Really good explanation and really useful. Thank you so much!!!! 

@tomleslie