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Vivek Balakrishnan Grade: 11
        

Can you please help me to solve with steps this type of problems?


Q:the 20th term of the series 2*4+4*6+6*8+........is


a)1600   b)1680   c)420   d)840.


 

6 years ago

Answers : (2)

manni stubtza
31 Points
										

1st takes it general term means Nth term which is as seen from sequence is 2n[2n+2]


u may verify it.....


on put n=1,2.... we get our 1st and 2nd term and so on....


hence for 20th term put n=20


so 2*20[2+2*20]=1680 which is our ans.


hence correct option is B

6 years ago
AKASH GOYAL AskiitiansExpert-IITD
419 Points
										

Dear Vivek


carefully look at the terms in each multiplication the two terms form  AP. like first terms are 2,4,6,8,.... with common difference 2 and first term being 2


20 th term= a+(n-1)d


               = 2+(20-1)2


              = 40


for the second term 4,6,8,..... common difference is 2 and first term being 4


using above formula


20th term= 4+(20-1)2 = 42


20 th term of given series = 40*42 = 1680


 


All the best.


AKASH GOYAL


AskiitiansExpert-IITD


 


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6 years ago
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