1.B
当x = 根号5/2 时, x*x-1/4 = 1,则有最大值 α = π/2;
当x = 0时,x*x-1/4 = -1/4 ,则有最小值 β = arcsin(-1/4);
得:
sin(α+β) = sin(π/2 + arcsin(-1/4))
= sin(π/2 - arcsin(1/4))
= cos(arcsin(1/4))
= 根号{1 - sin(arcsin(1/4))^2}
= 根号{1 - (1/4)^2}
= 根号{1 - 1/16}
= 根号15/4
2.C
lg|x|的值在[-1,1]之间的情况是x在[-10,10]
这个区间是sin(x+π/3)的3个周期多一点,画图可知这3个周期内有6个交点,当超出[-10,10]时lg|x|>1,不可能有交点
3.
做四边形的对角线,交于O
由交差弦定理(可能是这个名字,说交叉的弦互相分成比例)
得到两组相似三角形AOB DOC和AOD BOC
设AO为2x,有OB=3x,OC=6x,OD=4x
用余弦定理解三角形AOB AOD
(2x)^2+(3x)^2-cos(AOB)*2x*3x=2^2
(2x)^2+(4x)^2-cos(Pi-AOB)*2x*4x=4^2