A:树荫下圆形光斑
B:海市蜃楼
C:露珠下的叶脉
D:水中“倒影”
计算:(x-1)(x+2)=____
记实数中的最大数为max{x1,x2,...xn},最小数为min{x1,x2,...xn}.已知△ABC的三边边长为a,b,c(a≤b≤c),定义它的倾斜度为t=max{ab,bc,ca}·min{ab,bc,ca},则“t=1”是“△ABC为等边三角形”的( )
The children are making a snowman.They are____(happy/sad).