Do too much sum! Forget the basics?

Algebra Level 3

This is an easy problem in my school exams that killed most of my friends. Probably they are much concentrated on harder problems. Do you remember the basics?

S n = ( 1 ) ( 3 ) + ( 2 ) ( 4 ) + ( 3 ) ( 5 ) + . . . + n ( n + 2 ) \large S_n=(1)(3)+(2)(4)+(3)(5)+...+n(n+2)

What is the last three digit of S 30 S_{30} ?

You may use a calculator.


The answer is 385.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

4 solutions

Note that n ( n + 2 ) + 1 = ( n + 1 ) 2 n(n+2) + 1 = (n+1)^2

S n = 1 3 + 2 4 + 3 5 + + n ( n + 2 ) S_n = 1*3 + 2*4 + 3*5 + \ldots + n*(n+2)

S n = 1 3 + 1 1 + 2 4 + 1 1 + + n ( n + 2 ) + 1 1 \Rightarrow S_n = 1*3 + 1 - 1 + 2*4 + 1 - 1 + \ldots + n*(n+2) + 1 - 1

S n = 2 2 1 + 3 2 1 + + ( n + 1 ) 2 1 \Rightarrow S_n = 2^2 - 1 + 3^2 - 1 + \ldots + (n+1)^2 -1

S n = 2 2 + 3 2 + + ( n + 1 ) 2 n \Rightarrow S_n = 2^2 + 3^2 + \ldots + (n+1)^2 - n

S n = 1 2 + 2 2 + 3 2 + + ( n + 1 ) 2 ( n + 1 ) \Rightarrow S_n = 1^2 + 2^2 + 3^2 + \ldots + (n+1)^2 - (n+1)

S n = ( n + 1 ) ( n + 2 ) ( 2 n + 3 ) 6 ( n + 1 ) \Rightarrow S_n = \dfrac{(n+1)(n+2)(2n + 3)}{6} - (n+1)

Putting in n = 30 n = 30

S 30 = 10416 31 = 10385 S_{30} = 10416 - 31 = 10385

Therefore answer 385 \boxed{385}

The good old basic ( a + b ) ( a b ) = a 2 b 2 (a+b)(a-b)=a^2-b^2 , where b b is half of the difference between the two numbers.

Sharky Kesa - 7 years, 2 months ago

splendid.........

Mayankk Bhagat - 7 years, 2 months ago

u indeed have a genius brain

Mayankk Bhagat - 7 years, 2 months ago

Beautiful question

U Z - 6 years, 3 months ago
Kartik Tyagi
Mar 12, 2014

just open the bracket, n(n+2)

n 2 n^{2} + 2n

apply the summition

i = 0 n \sum_{i=0}^n n 2 n^{2} + i = 0 n \sum_{i=0}^n 2n

i = 0 n \sum_{i=0}^n n 2 n^{2} + 2 i = 0 n \sum_{i=0}^n n

n ( n + 1 ) ( 2 n + 1 ) 6 + 2 n ( n + 1 ) 2 \frac{n(n+1)(2n+1)}{6} + 2 \frac{n(n+1)}{2}

Now just put n=30

you will get 10385

so the ans is 385 \boxed{385}

Was that "you can use a calculator" lust to trick us??

Satvik Golechha - 7 years, 3 months ago

Log in to reply

The calculator is not much important though... Just some people like me use calculator to multiply 30 × 31 × 61 30\times31\times61 haha

Christopher Boo - 7 years, 3 months ago

Log in to reply

then i m like you

Kartik Tyagi - 7 years, 3 months ago
Bleach Byakuya
Jun 29, 2014

Lol ... just summation of n^2 + 2n from n=1 to 30 ...

Aronas Nuresi
Apr 15, 2014
1 3
2 11 8
3 26 15 7
4 50 24 9 2
5 85 35 11 2

a 1 3 + b 1 2 + c 1 + d = 03 a \cdot 1^3+b \cdot 1^2+c \cdot 1+d=03

a 2 3 + b 2 2 + c 2 + d = 11 a \cdot 2^3+b \cdot 2^2+c \cdot 2+ d=11

a 3 3 + b 3 2 + c 3 + d = 26 a \cdot 3^3+b \cdot 3^2+c \cdot 3+d=26

a 4 3 + b 4 2 + c 4 + d = 50 a \cdot 4^3+b \cdot 4^2+c \cdot 4+d=50

a = 1 3 , b = 3 2 , c = 7 6 , d = 0 a=\frac{1}{3}, b=\frac{3}{2}, c=\frac{7}{6}, d=0

a n = 1 3 n 3 + 3 2 n 2 + 7 6 n a_n=\frac{1}{3}n^3+\frac{3}{2}n^2+\frac{7}{6}n

a n = n 6 ( 2 n 2 + 9 n + 7 ) a_n=\frac{n}{6}(2n^2+9n+7)

a 30 = 10385 a_{30}=10385

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...