If $a,x_1,x_2,x_3\mathrm{...}$  and $b,y_1,y_2\mathrm{...}$ form two infinite A.P’s with common difference p and q respectively then the locus of $P(h,k)$  when $h=\frac{x_1+x_2+x_3\mathrm{...}+x_n}{n}$ , $k=\frac{y_1+y_2+\mathrm{....}+y_n}{n}$  is