π/4＜α＜3π/4,-π/2＜π/4-α＜0cos(π/4-α)=3/5sin(π/4-α)=-4/50＜β＜π/4π/2＜3π/4-β＜3π/4sin(3π/4-β)=5/13cos(3π/4-β)=-12/13cos(α+β)=-cos[π-(α+β)]=-cos[(π/4-α)+(3π/4-β)]=-[cos[(π...