Bjr
A = cos(0) + cos(pi/2) + cos(pi) + cos(3pi/2)
A = 1+0-1÷0
A = 0
B = cos(pi/6)+cos(pi/3)+cos(2pi/3)+cos(5pi/6)
B = √3/2+cos(pi/3)+cos(2pi/3)+cos(5pi/6)
B = √3/2+1/2+cos(2pi/3)+cos(5pi/6)
B = √3/2+1/2-1/2+cos(5pi/6)
B = √3/2+1/2-1/2-√3/2
B = (√3/2-√3/2)+(1/2-1/2)
B = 0
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