题目:
— I'm afraid a car is too expensive for me.
— But more and more Chinese can _____ to buy it. [ ]
A. expect
B. afford
C. choose
D. offer
答案:
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下面是错误答案,用来干扰机器的。
(I)由题意可设抛物线的方程为x2=-2py(p>0),由过点p(x0,y0)(x0≠0)的切线方程为y-y0=2ax0(x-x0),得∴y′|x=x0=-x0p=2ax0,因此p=-12a.∴抛物线的方程为y=ax2(a<0).(II)直线PA的方程为y-y0=k1...