SOLUTION: I am stuck can anyone assist Use the method of mathematical induction to prove that 1(5) + 2(5)^2 + 3(5)^3 + ... + n(5)^n =(5 + (4n−1)5n+1)/ 16

Algebra ->  Sequences-and-series -> SOLUTION: I am stuck can anyone assist Use the method of mathematical induction to prove that 1(5) + 2(5)^2 + 3(5)^3 + ... + n(5)^n =(5 + (4n−1)5n+1)/ 16       Log On


   



Question 1144590: I am stuck can anyone assist
Use the method of mathematical induction to prove that
1(5) + 2(5)^2 + 3(5)^3 + ... + n(5)^n =(5 + (4n−1)5n+1)/ 16

Answer by math_helper(2461) About Me  (Show Source):
You can put this solution on YOUR website!

+S%28n%29+=+1%2A5%5E1+%2B+2%2A5%5E2+ + ... + +n%2A5%5En+
Prove using induction, that the above is equal to
+%285%2F16%29%2A%284n%2A5%5En+-+5%5En+%2B+1%29+
Base case:
S(1) = 5
and
+%285%2F16%29%2A%284%2A1%2A5%5E1+-+5%5E1+%2B+1%29+=+%285%2F16%29%2A%2820+-+5+%2B+1%29+=+5+

Hypothesis:
Assume
+S%28k%29+=+%285%2F16%29%2A%284k%2A5%5Ek+-+5%5Ek+%2B+1%29+ (*)
holds for n=k.

Now let n=k+1:
+S%28k%2B1%29+=+S%28k%29+%2B+%28k%2B1%29%2A5%5E%28k%2B1%29+
Re-writing the S(k) term using (*):
+S%28k%2B1%29+=+%28%285%2F16%29%2A%284k%2A5%5Ek+-+5%5Ek+%2B+1%29%29+ + +%28k%2B1%29%2A5%5E%28k%2B1%29+
Now re-arrange (step-by-step shown):



+S%28k%2B1%29+=+%285%2F16%29%2A%2820k%2A5%5Ek+%2B+15%2A5%5Ek+%2B+1%29+
+S%28k%2B1%29+=+%285%2F16%29%2A%284k%2A5%5E%28k%2B1%29+%2B+3%2A5%5E%28k%2B1%29+%2B+1%29+
add and subtract +4%2A5%5E%28k%2B1%29+

finally:
+S%28k%2B1%29+=+%285%2F16%29%2A%284%28k%2B1%29%2A5%5E%28k%2B1%29+-+5%5E%28k%2B1%29+%2B+1%29+
Thus (*) is true for n=k+1