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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.
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
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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