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Ask iit jee aieee pet cbse icse state board community Community Discussion Question: sequences....
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shub (38)

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sum to 10 terms of the series 1.2.3+2.3.4+3.4.5.......=?

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joyfrancis (1504)

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here is the general method, lets say you have n terms instead of 10, you can always subst the value of n at the end.
 
The general term of the series is
=r(r+1)(r+2)
=r3+3r2+2r
 
Now substitute r as 1,2,3....n to get the sum of the series.
.: Sn = (1)3 + 3(1)2 + 2(1) + (2)3 + 3(2)2 + 2(2).....n terms
= (13 + 23 + 33... n terms) + 3(12 + 22 + 32....n terms) + 2(1 + 2 + 3... n terms)
 
= using sum of cubes of first n nat nos.= n2(n+1)2/4
sum of squares of first n =n(n+1)(2n + 1) / 6  
sum of first n nat numbers = n(n+1)/2
 you get the sum of series as
 
= n(n+1)(n+2)(n+3) / 4
 
.: for n = 10
we get the sum as 4290

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karthik2007 (3733)

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Excellent, and simple solution... I need more help on sequences and series...

Will nip in at times to solve problems :)
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