Answers

2015-11-30T20:16:36+05:30
It can be done using the formula n(n+1)(2n+1)/6
where n is the last natural number
sum = 20(20+1)(40+1)/6
         =2780



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1 5 1
2015-11-30T20:17:39+05:30
Squares of numbers are
1 - 1
2 - 4
3 - 9
4 - 16
5 - 25
6  - 36
7 - 49
 8 - 64
9 - 81
10 - 100
11 - 121
12 - 144
13 - 169
14 - 196
15 - 225
16 - 256
17 - 289
18 - 324
19 -361
20 -  400

sum of squares is 1+4+9+16+25+36+49+64+81+100+121+141+169+196+225+256+289+324+361+400
= 2867

0
or by The sum of the first N square numbers is: 
N*(N+1)*(2N+1)/6 
So putting N = 20 gives 2870.