If and be the roots of the equation , then what is the value of the sum(given ) :
...... ?
[Here ln(.) means natural logarithm, that is log to the base e]
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Consider the following infinite series:
1 + z 1 = 1 − z + z 2 − z 3 + …
Where ∣ z ∣ < 1
Integrating both sides from 0 to z gives:
ln ( 1 + z ) = z − 2 z 2 + 3 z 3 − 4 z 4 + …
Where ∣ z ∣ < 1
Now the given series can be rewritten as:
S = ( ( α x ) − 2 ( α x ) 2 + 3 ( α x ) 3 − … ) + ( ( β x ) − 2 ( β x ) 2 + 3 ( β x ) 3 − … )
Now, the pieces of information missing in the problem statement are that ∣ α x ∣ < 1 and ∣ β x ∣ < 1 . Assuming that these two conditions hold, the above-derived formulas are applied, leading to:
S = ln ( 1 + α x ) + ln ( 1 + β x )
S = ln ( 1 + ( α + β ) x + α β x 2 )
Using the given information in the problem statement, it can be deduced that the sum and product of roots of the given quadratic equation are:
α + β = − p α β = q
Therefore, the result is finally,
S = ln ( 1 − p x + q x 2 )
The information which needs to be added to the problem statement are the conditions that ∣ α x ∣ < 1 and ∣ β x ∣ < 1 .