问题 解答题

下面是马虎同学在一次测验中解答的填空题:①若x2=a2,则x=a;②方程2x(x-1)=x-1的解为x=1;③若x4-2x2-3=0,令x2=a,则a=3或-1.④经计算整式x+1与x-4的积为x2-3x-4,则一元二次方程x2-3x-4=0的所有根是x1=-1,x1=4.则其中答案完全正确的题目为______(将正确结论的序号填写在横线上)

答案

①x2=a2⇒x=±a故错;

②2x(x-1)=x-1即2x(x-1)-(x-1)=0

∴(2x-1)(x-1)=0

解得x=1或

1
2
故错;

③a2-2a-3=(a-3)(a+1)=0

解得a=3或-1;

但因为a=x2≥0,因此a=3故错;

④x2-3x-4=(x+1)(x-4)=0

解得x1=-1,x1=4故对;

故本题答案为④.

单项选择题

Six years later, in an about-face, the FBI admits that federal agents fired tear gas canisters capable of causing a fire at the Branch Davidian compound near Waco, Texas in 1993. But the official said the firing came several hours before the structure burst into flames, killing 80 people including the Davidians’ leader, David Koresh.

"In looking into this, we’ve come across information that shows some canisters that can be deemed pyrotechnic in nature were fired--hours before the fire started," the official said. "Devices were fired at the bunker, not at the main structure where the Davidians were camped out."

The Federal Bureau of Investigation maintains it did not start what turned to be a series of fiery bursts of flames that ended a 51-day standoff between branch members and the federal government. "This doesn’t change the bottom line that David Koresh started the fire and the government did not," the official said. "It simply Shows that devices that could probably be flammable were used in the early morning hours."

The law enforcement official said the canisters were fired not at the main structure where the Davidian members were camped out but at the nearby underground bunker. They bounced off the bunker’s concrete roof and landed in an open field well, the official said. The canisters were fired at around 6 a.m. , and the fire that destroyed the wooden compound started around noon, the official said. The official also added that other tear gas canisters used by agent that day were not flammable or potentially explosive.

While Coulson denied the grenades played a role in starting the fire, his statement marked the first time that any U.S. government official has publicly contradicted the government’s position that federal agents used nothing on the final day of the siege at Waco that could have sparked the fire that engulfed the compound. The cause of the fiery end is a major focus of an ongoing inquiry by the Texas Rangers into the Waco siege.

Coulson’s speech has()

A. made the matter even more complicated and confusing

B. been approved by the government

C. met sharp criticism

D. brought the whole matter into broad daylight

多项选择题