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+........isa)1600   b)1680   c)420   d)840.

manni stubtza
31 Points
13 years ago

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

420 Points
13 years ago

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

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