ans
15311
$\dfrac\pi 4$

15312
$2\sqrt{5}$

15313
$x+2y-3=0$

15314
$8$

15315
$6$

15316
$m<-1$

15317
$3$

15318
$-\dfrac 34$

15319
$2$

15320
$x^2+y^2=16$

15321
$\dfrac{R-r}{R+r}$

15322
$1$

15323
D

15324
B

15325
A

15326
D

15327
(1) $\dfrac 13$; (2) $-1$或$2$

15328
(1) $3x+4y=25$; (2) $x=3$或$7x-24y+75=0$

15329
(1) $y^2=4x$; (2) $\sqrt{5}$

15330
(1) $\dfrac{x^2}8-\dfrac{y^2}4=1$; (2) $\dfrac{(2x-3)^2}8-y^2=1$; (3) $3-2\sqrt{2}$

15331
(1) $\dfrac{x^2}4+\dfrac{y^2}3=1$; (2) $\sqrt{3}$; (3) $(\dfrac 43,0)$