cos5=cos(-5)=cos(2π-5)=sin[π/2-(2π-5)]=sin(5-3π/2)5-3π/2≈0.29则0<岩历念1/5<5-3π/2<π/2在(0,π/2),sinx是增函数粗困所烂历以sin1/5sin21π/5=sinπ/5tan19π/8=tan3π/8sin42π/5=sin2π/53π/8>π/4所以tan3π/8>1由sinx单调性sinπ/5所以sin21π/5