(x+1)(x-3)=12.
方程可变形为:x2-2x-15=0,
分解因式得:(x-5)(x+3)=0,
则:x-5=0,x+3=0,
解得:x1=5,x2=-3.
人口自然增长率是一定时期内人口自然增长数与该时期().
A.年初人口之比
B.年平均人口之比
C.年终人口之比
D.调查时人口之比
E.总人口之比
Greg sells gaskets. On three sales, Greg has received commissions of $385, $70, and $190, and he has one additional sale pending. If Greg is to receive an average (arithmetic mean) commission of exactly $220 on the four sales, then the fourth commission must be:().
A. $135
B. $155
C. $220
D. $235
E. $645