15, (1) is symmetrical ABout x, then ab must be perpendicular to the x axis, so b= 1 2a=-3 a=-2/3.

(2) Similarly -2b=2 b=- 1 2a=3 a=2/3.

16，P 1(-a，b) P2(-a，-b)

So -a=-3 a=3 -b=4 b=-4.

So a=3 b=-4