If three distinct numbers a ,   b ,   c are in G.P. and the equations a x 2 + 2 b x + c = 0 and d…

If three distinct numbers a, b, c are in G.P. and the equations ax2+2bx+c=0 and dx2+2ex+f=0 have a common root, then which one of the following statements is correct?
  1. da,eb,fc are in A.P.
  2. d, e, f are in A.P.
  3.  d, e, f are in G.P.
  4. da,eb,fc are in G.P.

Solution

a,b,c are in G.P. b2=ac (i)
Equation ax2+2bx+c=0
Hence, D=4b2-4ac=0 b2=ac
equation has equal roots roots are =-ba,-ba ( sum of roots α+α=-2ba )
 Both equations have one root in common.
 Common root is -ba. It will satisfy equation.
d-ba2+2e-ba+f=0db2-2aeb+a2f=0
dac-2aeb+a2f=0 b2=ac
dc-2aeb+af=0
dcac-2ebac+afac=0 (dividing by ac )
da-2ebb2+fc=-da+fc=2eb
da,eb,fc are in A.P.

Asked in: JEE Main 2019 (08 Apr Shift 2)

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