a是第二象限的角,则π/2+2kπ＜a＜π+2kπ所以π/4+kπ＜a/2＜π/2+kπ当k是偶数时,令k=2n,则π/4+2nπ＜a/2＜π/2+2nπ此时a/2在第一象限,tan a/2＞sin a/2＞cos a/2当k是奇数时,令k=2n+1,则5π/4+2nπ＜a/2＜3π/2...