八年级外研英语周报2022答案

21.解:(1)设g(x)=f(x)-e2=(x-1)e2-ax2U g(x)=xe- 2ar=z(e-2a)当a≤0时,令g(x)>0,得x>0,令g(x)<0,得x<0,所以g(x)在(-∞,0)上单调递减,在(0,+∞)上单调递增;(2分)当0 0,得x<1n2a或x>0,令g(x)<0,得1n2a 0恒成立,所以g(x)在R上单调递增;(4分)当a>时,令g(x)>0,得x<0或x>ln2a令g(x)<0,得0 时,g(x)在(-∞,0)和(2,+∞)上单调递增,在(0,i2a)上单调递减(6分x≥0得aT(2)由f(x≥0当x>0时,e-x2-ax-1≥0,即a≤e-x2-1对Vx>0恒成立(7分)设h(x)则h(x)=(x=1)(c-x设q(x)=e2-x-1,则g(x)=c-1因为x>0,所以p(x)>0,可知g(x)在(0,+∞)上单调递增,所以g(x)>g(0)=0,即e2>x+1,(10分)所以h(x)在(0,1)上单调递减,在(1,+∞)上单调递增,可得h(x)≥h(1)=e-2,所以a≤e-2,所以实数a的取值范围是(-∞,e-2](12分)

应用文写作Dear BryanI sincerely invite you to our home to celebrate the Spring Festival together. It Is the most importantraditional festival for Chinese people No matter how far away we are, we'll try to get home as it is a time iorfamily reunion. We'll cook a lot of traditional Chinese food, such as dumpling, fish, meat, chicken, vegeta blesand fruits. If it is convenient for you, you can join us in our great dinner on Spring Festivals Eve. You are sureto have a good timeAr midnight, we'll fire fireworks. The foilowing morning, we'll visit elderly family members and expressrespect to themLooking forward to your replYours,

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