# Thread: another weird maths question!!

1. Tabber
Join Date
Apr 2005
Location
england
Posts
242

## another weird maths question!!

ok, here we go agen...
maths has once again succeded in leading me 2 a dead end
here is the problem:

prove that if:
f(r)=r!, then f(r+1) - f(r)=r*r!

where f(r) is a function and * means multiply

then the next part is to sum the series:
1*1! + 2*2! + 3*3! ........ + n*n!
i think the summing part may require the difference method ( i can explain that if need be in a later post), however i could be wrong

if ne1 knows how to prove it, i would be grateful, thanks in advance
also i will be givin reps 2 those who help constructively

2. f(r+1)=(r+1)!=(r+1)r!=(r+1)f(r)
so f(r+1)=(r+1)f(r)
=> f(r+1)-f(r)=r*f(r)
=>f(r+1)-f(r)=r*r!

now, for nth value of the series...

f(n+1)-f(n)=n*n!

so entering values as n=1,2,3....

f(2)-f(1)=1*1!
f(3)-f(2)=2*2!
f(4)-f(3)=3*3!
.
.
.
.
.
f(n)-f(n-1)=(n-1)*(n-1)!
f(n+1)-f(n)=n*n!
----------------------------
f(n+1)-f(1)=the required sum
(n+1)!-1 (not sure abt this one,i.e., is 1!=1? almost 6 yrs ago I used to do it)
Inform if it solves the prob

3. Tabber
Join Date
Apr 2005
Location
england
Posts
242
i am impressed with the speed of your response , u got the rite answer, the book says so, lol
but i dont really get how u went through it, with all the different stages. cud u explain the stages u went through?

4. which part u didn't get ? the proof part or the addition part?

5. Pagol pagol!!!Wot A GR8 ANSWER! Subho da, you are gr8 man!

6. Originally Posted by mr singh
ok, here we go agen...
maths has once again succeded in leading me 2 a dead end
here is the problem:

prove that if:
\f(r)=r!, then f(r+1) - f(r)=r*r!

where f(r) is a function and * means multiply

then the next part is to sum the series:
1*1! + 2*2! + 3*3! ........ + n*n!
i think the summing part may require the difference method ( i can explain that if need be in a later post), however i could be wrong

if ne1 knows how to prove it, i would be grateful, thanks in advance
also i will be givin reps 2 those who help constructively
fine brother nice question but easy one

ok f(r+1)-f(r)=r+1!-r!
=(r+1)*r!-r!
=(r+1-1)r!
=r*r!

7. Tabber
Join Date
Apr 2005
Location
england
Posts
242
man, i jus gettin more n more confused... i need sum serious wrk on my function notation, :(... im so stupid
god, these 2 days of schl i missed through illness hav made me an idiot :(

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