# How many terms of the series 2,4,6,......must be taken in order that the sum may be 420?

2
by chokeh

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by chokeh

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hence we have to take this series to number 40 and terms 20 to obtain 420.

d=2

Sn=420

we have to find n

Sn=n/2 (2a+(n-1)d)=420 a=2

n( 4 + (n-1) 2)=840 d-2

n(4 + 2n-2) =840

n( 2n+2) = 840

2n²+2n -840=0

n²+n-420=0

n=20,-21

since n is positive number of terms needed are 20

so if we take 20 terms of the AP=2,4,6

we could get the sum 420