2 × 3 x 2 + 3 × 4 2 x 3 + 4 × 5 3 x 4 + …
For ∣ x ∣ < 1 , the series above converge to ( p − x q ) ln ( 1 − x ) − q .
Find the value of p 4 + q 4 .
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Consider the series -
1 + x + x 2 + x 3 + x 4 + . . . . . . . = 1 − x 1
Differentiating and multiplying by x ,
x + 2 x 2 + 3 x 3 + 4 x 4 + . . . . . = ( 1 − x ) 2 − x
Integrating,
2 x 2 + 3 2 x 3 + . . . . = ∫ ( 1 − x ) 2 − x
Using u-substitution as u = 1 − x ,
2 x 2 + 3 2 x 3 + . . . . = ∫ u 2 1 − u
= ∫ u 2 1 − ∫ u 1
2 x 2 + 3 2 x 3 + . . . . = − ( 1 − x 1 + l n ( 1 − x ) )
Again integrating,
3 . 2 x 3 + 4 . 3 2 x 4 + . . . . = − ∫ 1 − x 1 − ∫ l n ( 1 − x )
Once again using u-substitution and IBP,
3 . 2 x 3 + 4 . 3 2 x 4 + . . . . = l n ( 1 − x ) + ( 1 − x ) l n ( 1 − x ) + x
3 . 2 x 3 + 4 . 3 2 x 4 + . . . . = ( 2 − x ) l n ( 1 − x ) + x
Dividing by x,
3 . 2 x 2 + 4 . 3 2 x 3 + . . . . . = ( x 2 − 1 ) l n ( 1 − x ) + 1
p 4 + q 4 = 2 4 + 1 4 = 1 7
I did the same, but how is the ans you got equal to ( 1 − x 2 ) ( l n ( 1 − x ) ) − 2
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first, in summation form n = 1 ∑ ∞ ( n ( n + 1 ) ( n − 1 ) x n ) now, factor = = = = = n ( n + 1 ) ( n − 1 ) x n n ( n + 1 ) n x n − n ( n + 1 ) x n n + 1 x n − x n ( n ( n + 1 ) 1 ) n + 1 x n − x n ( n 1 − n + 1 1 ) 2 n + 1 x n − n x n hence n = 1 ∑ ∞ ( n ( n + 1 ) ( n − 1 ) x n ) = 2 n = 1 ∑ ∞ ( n + 1 x n ) − n = 1 ∑ ∞ ( n x n ) by a Maclaurin series expansions, n = 1 ∑ ∞ ( n x n ) = − lo g ( 1 − x ) and = = = = = = ∑ n = 1 ∞ ( n + 1 x n ) 2 x + 3 x 2 + 4 x 3 . . . . . . x 2 x 2 + 3 x 3 + 4 x 4 . . . x x + 2 x 2 + 3 x 3 + 4 x 4 . . . . . − x x ∑ n = 1 ∞ ( n x n ) − 1 − x lo g ( 1 − x ) − 1 hence 2 n = 1 ∑ ∞ ( n + 1 x n ) − n = 1 ∑ ∞ ( n x n ) = 2 ( − x lo g ( 1 − x ) − 1 ) + lo g ( 1 − x ) = ( 1 − x 2 ) lo g ( 1 − x ) − 2 and 1 4 + 2 4 = 1 7 . the question must mention that ∣ x ∣ < 1 and i dont see what is so algebric 'bout it, more of a calculus problem.