Tính
a/ \(\frac{1}{90}-\frac{1}{72}-\frac{1}{56}-\frac{1}{42}-\frac{1}{30}-\frac{1}{20}-\frac{1}{12}-\frac{1}{6}-\frac{1}{2}\)
b/ A= \(\left(\frac{1}{2^2}-1\right).\left(\frac{1}{3^2}-1\right)....\left(\frac{1}{2013^2}-1\right).\left(\frac{1}{1014^2}-1\right)\)
B=\(\frac{-1}{2}\)
So sánh A và B