【sin330°*tan(-13π/3)】/【cos(-19π/6)*cos690°】
问题描述：
【sin330°*tan(-13π/3)】/【cos(-19π/6)*cos690°】
答
y=x^2-5x+25/4-25/4+6
=(x-5/2)^2-1/4
开口向上
所以在对称轴左边递减
所以是(-∞,5/2)
Sn=(a1+an)n/2
Sn/n=(a1+an)/2
=[a1+a1+(n-1)d]/2
=a1+(d/2)(n-1)
所以是以a1为首项，d/2为公差的等差数列
答
原式=sin(-30)tan(-π/3)/[cos(5π/6)cos(-30)]
=(-1/2)(-√3)/[(-√3/2)(√3/2)
=2√3/3