题目

化简：sin(x+60°)+2sin(x-60°)-cos(120°-x). 答案：解:原式=sin(x+60°)-cos［180°-(60°+x)］+2sin(x-60°)=sin(x+60°)+cos(60°+x)+2sin(x-60°)=2［sin(x+60°)+cos(60°+x)］+2sin(x-60°)=2［cos60°sin(x+60°)+sin60°cos(60°+x)］+2sin(x-60°)=2sin(x+120°)+2sin(x-60°)=2sin(180°-60°+x)+2sin(x-60°)=-2sin(x-60°)+2sin(x-60°)=0.点评：本题运用“异角化同角”思路.如图是探究滑动摩擦力跟哪些因素有关的实验装置，用弹簧测力计匀速拉动同一长方体木块所做四次实验，其中甲和丙为同一水平木板表面，乙和丁为同一水平棉布表面．（1）观察图甲和乙得出的结论是______．（2）观察图甲和丙得出的结论是______．