∵0＜α＜π/2∴π/3＜α+π/3 ＜5π/6又∵cos(π/3 +α)=-3/5 ＜ 0∴π/2＜α+π/3 ＜5π/6sin(π/3 +α)=4/5∵π/2＜β＜π∴-π/3＜2π/3 -β＜π/6又∵sin(2π/3 -β)=5/13 ＞ 0∴0＜2π/3 -β＜π/6cos(2π/3 -...