设0设0
设01/cost + a/(1-cos t)=16,1-cos t+acost=16cost-16cos²t,16cos²t+(a-17)cost+1=0,Δ=(a-17)²-64≥0,a≥25或a≤9,∵0
("1"的代换)“cost+1-cost=1"f(t)=(1/cos t + a/(1-cos t))(cost+1-cost) 接下去用不等式