a = 1/15 + 1/35 + 1/63 + 1/99 + 1/143 + 1/195 + 1/255 + 1/323
a = 1/3x5 + 1/5x7 + 1/7x9 + 1/9x11 + 1/11x13 + 1/13x15 + 1/15x17 + 1/17x19
2xa = (1/3-1/5) + (1/5-1/7) + (1/7-1/9) + (1/9-1/11) + (1/11-1/13) + (1/13-1/15) + (1/15-1/17) + (1/17-1/19)
2xa = 1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 + 1/9 - 1/11 + 1/11 - 1/13 + 1/13 - 1/15 + 1/15 - 1/17 + 1/17 - 1/19
2xa = 1/3 - 1/19
2xa = 19/57 - 3/57
2xa = 16/57
a = 16/57 : 2
a = 8/57.