252525/525525*252252/525252
=25×10101/525×1001 ×252×1001/52×10101
=25×252/525×52
=3/13
2又1/3+4又1/15+6又1/35+8又1/63
=2+4+6+8+1/3+1/15+1/35+1/63
=20+1/2(1-1/3+1/3-1/5+1/5-1/7+1/7-1/9)
=20又4/9