Answers

2014-09-11T14:21:09+05:30
2+4+6............38+40=420
hence we have to take this series to number 40 and terms 20 to obtain 420.

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2014-09-11T14:27:39+05:30
Ap=2,4,6
  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


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