as x,y,z are in HP yhen 1/x , 1/y , 1/z will be in AP
let the first term & common diff on the AP series be a,d resp,
then
1/x = a+(p-1)d .....(i)
1/y = a+(q-1)d.......(ii)
1/z = a+(r-1)d........(iii)
(1/y) - (1/z) = (q-r)d => (q-r)d/x = (1/xy)-(1/xz) .......(a)
simlarly,
(r-p)d/y = (1/yz)-(1xz)......(b)
(p-q)d/z = (1/xz) - (1/yz)........(c)
adding (a),(b),(c) we get req. result
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