18.已知数列{an}前n项和为Sn=n2-2n+a(a∈R,n∈N*),若该数列是等差数列则a的值为( )
| A. | 0 | B. | 1 | C. | -1 | D. | 不确定 |
16.已知随机变量X:B(20,$\frac{1}{3}$),要使P(X=k)的值最大,则k=( )
| A. | 5或6 | B. | 6或7 | C. | 7 | D. | 7或8 |
14.已知f($\frac{1}{x}$)=$\frac{x}{x+1}$,则f′(x)=( )
0 225492 225500 225506 225510 225516 225518 225522 225528 225530 225536 225542 225546 225548 225552 225558 225560 225566 225570 225572 225576 225578 225582 225584 225586 225587 225588 225590 225591 225592 225594 225596 225600 225602 225606 225608 225612 225618 225620 225626 225630 225632 225636 225642 225648 225650 225656 225660 225662 225668 225672 225678 225686 266669
| A. | $\frac{1}{1+x}$ | B. | -$\frac{1}{1+x}$ | C. | $\frac{1}{(1+x)^{2}}$ | D. | -$\frac{1}{(1+x)^{2}}$ |