8th January 2009

lets take a simple one..

what is the remainder when

(1+2+3+.... n)n is divided by n-1?

17 Answers

13
MAK ·

another simple objective method... [6]

substitute n={ur lucky no.*} [3]............ (*make sure dat it is as small as possible) [4]

1
Honey Arora ·

ok........

62
Lokesh Verma ·

think of this one

what is the remainder when

(1+a)n = 1+a.{polynomial of a}

hence when divided by a, the remainder will be 1

more generally

(x+a)n = xn+a.{polynomial of a}

hence when divided by a, the remainder will be same as remiander of xn on division by a

13
MAK ·

applying binomial expansion... every term of the expansion (1+Σ(n-1) )n will be divisible by (n-1) except the first term which is nC0.1n = 1... so the remainder will be the remainder obtained by dividing 1 with n-1...

1
Honey Arora ·

nd itz raised to power n.........won't tht matter?

62
Lokesh Verma ·

Honey..

what is the remainder when 3 is divided by 2...

go back a few classes.. when we did not study fractions..

when we used to do quotient and remainder....

I know why u are confused :)

Or if this is not ur doubt.. then please re ask..

13
MAK ·

@honey...

now after concluding dat d remainder is 1/n-1 ... find d remainder when 1 is divided by n-1... since remainder must not have divisor in its denominator(basic condition) ... so 1=(n-1).0 + 1... hence remainder is one...

hope u got it now... [1]

1
skygirl ·

yeah mak u are correct ,,, i chceked that way only....

aur agar isme options rehta... toh solve bhi nahi karte.. :P put kar dete :P

par nishant bhaiya is obviously intelligent enuf to make these all subjective :P

1
Honey Arora ·

bt whn u divide........isn't it whole [1 + Σ(n-1)]/n-1
so hw cld u get remainder 1?????

in tht way remainder shd be 1/n-1

13
MAK ·

[nn.(n+1)n / 2n] / (n-1)

[nn.(n-1+2)n / 2n] / (n-1)

expanding and dividing with n-1 implies remainder of d entire term = reminder of nn/n-1...

(n-1+1)n / n-1 .... simplifying and dividing gives remainder of given term = remainder of 1/n-1 which is 1...

let me know if i'm wrong...

1
skygirl ·

yeah 1 :)

62
Lokesh Verma ·

honey .. maq is right :)

If you dont understand this one.. do try to understand it .. it is not very complex.. but very useful indeed !

13
MAK ·

y can't I...?

are u asking for explanation or arguing...?

1
Honey Arora ·

hw cld u solve tht way?

13
MAK ·

yeah got it... 1+2+3+...+n = 1 + Σ(n-1)...

since Σ(n-1) is divisible by n-1... the remainder is 1... [1]

13
MAK ·

more simpler one...!!! [11]

hmmm... let me try... [12]

62
Lokesh Verma ·

hmm.. this is good.. but did u make a simpler solution complex?

I mean there is a still more simpler solution :)

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