Dear Student,
line passing through (a,b) is y= mx-am+b, 
line normal to this and passing through 0,0:
y=-x/m
let foot of perpendicular be (h,k)
we know: k= mh-am+b and
m= -h/k,
=>k= (-h/k)*h + a(h/k) +b,
so=k2=-h2+ah+bk,
So h2+k2= ah+bk,
 therefore locus is x2+y2=ax+by [ans]