∵a、b为锐角,且sina＝8/17∴cosa＝√(1－sin²a)＝15/17sin(a－b)＝√[1－cos²(a－b)]＝20/29∴cosb＝cos[a－(a－b)]＝cosacos(a－b)＋sinasin(a－b)＝15/17×21/29＋8/17×20/29＝475/493...