∵π/4＜a＜3π/4,∴π/2＜π/4＋a＜π,∴sin（π/4＋a）＝4/5.∵0＜b＜π/4,∴3π/4＜3π/4＋b＜π,∴cos（3π/4＋b）＝－12/13.∴sin（a＋b）＝－sin［π＋（a＋b）］＝－sin［（π/4＋a）＋（3π/4＋b）］＝－［s...