Put them all in

Algebra Level 1

n = 1 10 n ( 1 + n + n 2 ) = ? \large \displaystyle\sum_{n=1}^{10}n\big(1+n+n^2\big)= \, ?


The answer is 3465.

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.

5 solutions

Akshat Sharda
Mar 10, 2016

n = 1 10 n ( 1 + n + n 2 ) = n = 1 10 n + n 2 + n 3 = 10 11 2 + 10 11 21 6 + ( 10 11 2 ) 2 = 55 + 385 + 5 5 2 = 3465 \begin{aligned} \displaystyle \sum^{10}_{n=1} n(1+n+n^2) & = \displaystyle \sum^{10}_{n=1}n+n^2+n^3 \\ & = \frac{10\cdot 11}{2}+\frac{10\cdot 11 \cdot 21}{6}+\left( \frac{10\cdot 11}{2}\right)^{2} \\ & = 55+385+55^2 \\ & = \boxed{3465} \end{aligned}


Formulae used: n = 1 k n = k ( k + 1 ) 2 n = 1 k n 2 = k ( k + 1 ) ( 2 k + 1 ) 6 n = 1 k n 3 = ( k ( k + 1 ) 2 ) 2 \text{Formulae used:} \\ \displaystyle \sum^{k}_{n=1}n=\frac{k(k+1)}{2} \\ \displaystyle \sum^{k}_{n=1}n^2=\frac{k(k+1)(2k+1)}{6} \\ \displaystyle \sum^{k}_{n=1}n^3=\left(\frac{k(k+1)}{2}\right)^2

akshat very easy question it is

Arun Garg - 5 years, 2 months ago

Nice solution

BTW, it would've been way cooler if the title would've been Put (Them all) in LOL (No offence to anyone)

Mehul Arora - 5 years, 3 months ago

Log in to reply

Can't understand! Please explain :-P

Akshat Sharda - 5 years, 3 months ago

Log in to reply

Understanding things is an art that comes from within :P

Mehul Arora - 5 years, 3 months ago

Log in to reply

@Mehul Arora L O L \mathfrak{LOL}

Akshat Sharda - 5 years, 3 months ago

same here C:

Jacopo Guidolin - 5 years, 3 months ago

I had the right idea but got confused with the algebra.

Ricaul Castellon Sanchez - 1 year, 7 months ago

Nice solution. Can you write out the formulas that you used for the sum of n n , the sum of n 2 {n}^{2} , and the sum of n 3 {n}^{3} ? It would make your process much more clear.

Andy Wong - 5 years, 3 months ago

Log in to reply

T H A N K S \mathcal{THANKS}

Akshat Sharda - 5 years, 3 months ago
Nikola Lozajic
Mar 10, 2016

If you know the formulae for the sums of n, n squared and n cubed, you can do it quite quickly.

Considering that the the sum of n(1+n+n^2) can be expanded to n+n^2+n^3, we can apply the following formulas:

Sum of n= (n (n+1))/2 Sum of n squares (n^3)/3 + (n^2)/2 + n/6 Sum of n cubes= ((n (n+1))/2)^2

We substitute: (10x11)/2 + 1000/3 + 100/2 + 10/6 + ((10x11)/2)^2

Which equals to 55 + 385 + 3025 = 3465

Liviu Vigu-Giurea
Feb 16, 2018

You rewrite the sum as: (k(k+1)/2)(1+(2k+1)/3 + k(k+1)/2), where k = 10 => 55(1+7+55) (= 55(60+3) = 165 + 3300)=3465 The steps in parentheses are for not doing any computer calculation.

Zlatko Durmis
Jan 2, 2018

= n = 1 10 n + n = 1 10 n 2 + n = 1 10 n 3 = 10 11 2 + 10 11 ( 2 10 + 1 ) 6 + ( 10 11 2 ) 2 = 55 + 385 + 5 5 2 = 440 + ( 5 6 100 + 25 ) = 440 + 3025 = 3465 =\sum_{n=1}^{10}n+\sum_{n=1}^{10}n^2+\sum_{n=1}^{10}n^3=\frac{10\cdot11}{2}+\frac{10\cdot11\cdot(2 \cdot 10+1)}{6}+\left(\frac{10\cdot 11}{2}\right)^2=55+385+55^2=440+(5\cdot6\cdot100+25)=440+3025=3465

Gokalbhai Vadi
Mar 11, 2016

10×11/2+{(10)×(10+1)×(10×2+1)}/6+(10^2)(10+1)^2/4 =55+385+3025 =3465

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...