It was difficult __________ something because it was dark.
A：see
B：to see
C：sees
D：seeing
①f（x1+x2）=f（x1）•f（x2）；
②f（x1•x2）=f（x1）+f（x2）；
③ f(x1)+f(x2)x1−x2 ＞0；
④ f(x1+x22)＜f(x1)+f(x2)2 ．
当f（x）=lgx时，上述结论中正确结论的序号是____．
如果x+yy=74，那么xy的值是（ ）
