用万能公式.0＜t＜∏,0＜t/2＜∏/2.则f（t）=1/cost+a/1-cost=(1+tan^2(t/2))/(1-tan^2(t/2))+a/2sin^2(t/2)=2/(1-tan^2(t/2))-1+(a/2)·(1+1/tan^2(t/2))令u=tan^2(t/2),则u的取值范围是(0,+∞)则f（t）=2/(1-u)-1+...