To prove: xb - c . yc - a . za - b = 1….(i)
It is given that a,b,c are in A.P.
⇒ 2b = a + c…(ii)
And x,y,z, are in G.P.
⇒ y2 = xz
⇒ x = y2 /z
Substitute this value of x in equation (i),we get
L.H.S =

Hence, proved that . If a, b, c are in AP and x, y, z are in GP then xb - c . yc - a . za - b= 1