a) A=(-1).(-1)2.(-1)3.(-1)4...(-1)2012.(-1)2013 b) B=70.(\(\frac{131313}{565656}+\frac{131313}{727272}+\frac{131313}{909090}\)) c) C=\(\frac{2a}{3b}+\frac{3b}{4c}+\frac{4c}{5d}+\frac{5d}{2a}\)biết \(\frac{2a}{3b}=\frac{3b}{4c}=\frac{4c}{5d}=\frac{5d}{2a}\)