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What is the sum of the first twenty terms that are in both series 3 + 7 + 11 + 15 + ... and 1 + 6 + 11 + 16 + ...

 Feb 17, 2021

Best Answer 

 #1
avatar+29193 
+2

S1     3 7 11 15 19 23 27 31 35 39 43 47 51

S2     1 6 11 16 21 26      31      36 41 46 51

 

makes a new series     11   31   51      a = 11   d = 20

S = (n/2) × (2a + (n−1)d)

 S20 = 20/2 ( 2(11)+19(20))

        = 10 (22+ 380) = 10(402) = 4020

 Feb 17, 2021
 #1
avatar+29193 
+2
Best Answer

S1     3 7 11 15 19 23 27 31 35 39 43 47 51

S2     1 6 11 16 21 26      31      36 41 46 51

 

makes a new series     11   31   51      a = 11   d = 20

S = (n/2) × (2a + (n−1)d)

 S20 = 20/2 ( 2(11)+19(20))

        = 10 (22+ 380) = 10(402) = 4020

ElectricPavlov Feb 17, 2021

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