Tính nhanh
1, A = \(\frac{4}{5\cdot7}\)+ \(\frac{4}{7\cdot9}\) + ..... + \(\frac{4}{59\cdot61}\)
2, B = \(\frac{3^2}{2\cdot5}\) + \(\frac{3^2}{58}\) + \(\frac{3^2}{8\cdot11}\) + ..... + \(\frac{3^2}{38\cdot41}\)
3, C = ( \(\frac{1}{2}\) + 1 ) . ( \(\frac{1}{3}\)+ 1 ) . ( \(\frac{1}{4}\)+ 1 ) . .... . ( \(\frac{1}{99}\)+ 1 )