Use the principle of induction to prove that
1.1! + 2.2! + ... + n.n! = (n+1)! - 1
for all n .
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Let P(k) be the statement that:
1.1! + 2.2! + ... + k.k! = (k+1)! -1, where k is a positive integer.
For k = 1, as 1! = 2! - 1, P(1) is obviously true.
Now suppose that P(n) is true.
Then 1.1! + 2.2! + ... + n.n! = (n+1)! -1.
Adding (n+1).(n+1)! to both sides, we have:
1.1! + 2.2! + ... + n.n! + (n+1)(n+1)!
= (n+1)! - 1 + (n+1)(n+1)!
= (n+1)! + (n+1)(n+1)! - 1
= (n+1)!{1 + (n+1)} - 1
= (n+2)! - 1
It follows that P(n+1) is true. Therefore, by induction, P(k) is true for all positive integer k.
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