6.
已知AD为△ABC的外接圆⊙O的直径,点E是△ABC的内心,AE的延长线交BC于点F,交⊙O于点D,BC=6,cos∠BAC=$\frac{4}{5}$,则EF的长是( )
| A. | 1 | B. | 4-$\sqrt{10}$ | C. | 5-$\sqrt{10}$ | D. | $\sqrt{10}$-1 |
4.若x2+kxy+16y2是一个完全平方式,那么k的值为( )
0 283504 283512 283518 283522 283528 283530 283534 283540 283542 283548 283554 283558 283560 283564 283570 283572 283578 283582 283584 283588 283590 283594 283596 283598 283599 283600 283602 283603 283604 283606 283608 283612 283614 283618 283620 283624 283630 283632 283638 283642 283644 283648 283654 283660 283662 283668 283672 283674 283680 283684 283690 283698 366461
| A. | 4 | B. | 8 | C. | ±8 | D. | ±16 |