The roots of the equation :
x
5
−
4
0
x
4
+
p
x
3
+
q
x
2
+
r
x
+
s
=
0
are in a
geometric progression
. The sum of their reciprocal is 10. Find the
absolute value
of
s
.
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Two mistakes in a single line. It should be " t 1 ( u 2 + u 1 + 1 + u − 1 + u − 2 ) = 1 0 "
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Since the roots are in geometric progression, we can write them as { t u − 2 , t u − 1 , t , t u , t u 2 } . Note that none of the roots can be zero (otherwise the reciprocals wouldn't be defined), so this representation is valid. The reason for this choice is it makes the algebra a little easier:
The sum of the roots is t ( u − 2 + u − 1 + 1 + u + u 2 ) = 4 0 (using Vieta).
The sum of the reciprocals of the roots is t 1 ( u 2 + u + 1 + u − 1 + u − 2 ) = 1 0 (given in the problem).
From these, we get t 2 = 4 , so that t = ± 2 .
The product of the roots (using Vieta again) is just t 5 = − s ; from above, this means that ∣ s ∣ = 2 5 = 3 2 .