2/1X3+2/3X5+2/5x7+.+2/19x21全过程（最好带原因）,

2/1X3+2/3X5+2/5x7+.+2/19x21全过程（最好带原因）, 2/1X3+2/3X5+2/5x7+.+2/19x21 (2X2)/1X3+(4X4)/(3X5)+(6X6)/(5X7)+...+(20X20)/(19X21)==? 1/1X3+1/3X5+1/5X7...1/17X19+1/19X21= 2/1x3+2/3X5+2/5x7=2/7x9+2/9x11 1+2x3+4x5+6x7+.50x51的和是奇数还是偶数?写出原因 1x2/1+2x3/1+3x4/1+4x5/1+5x6/1+6x7/1 1/1x2+1/2x3+1/3x4+1/4x5+1/5x6+1/6x7 已知1-1/3=2/1x3,计算1/1x3+1/3x5+1/5x7+1/7x9+1/9x11= 1/1X3 + 1/3X5 + 1/5X7 + .+ 1/(2x-1)(2x+1)=10/21(x为整数） 1+2X3+4X5+6X7+8X9=303 先阅读再计算2/1x3=1-1/32/3x5=1/3-1/52/5x7=1/5-7=1/7试计算1/1x3+1/3x5+