因式分解
(1)x(x+4)+3
(2)x2-2x+1-y2.
(1)原式=x2+4x+3
=(x+1)(x+3);
(2)原式=(x-1)2-y2
=(x+y-1)(x-y-1).
When I try to understand () that prevents so many Americans from being as happy as one might expect, it seems to me that there are two causes.
A. why it does
B. what it does
C. what it is
D. why it is
电力机车在自动过分相时,由()、()、()电力机车、钢轨等组成一个完整的封闭回路。