Question

sum the series 3+5+7_a20

Answer 1

Hi ,

3 , 5 , 7 ,... is an A. P

first term ( a ) = 3 ,

Common difference ( d ) = a2 - a1

d = 5 - 3 = 2

We know that ,

an = a + ( n - 1 )d

a20 = 3 + ( 20 - 1 ) 2

= 3 + 19 × 2

= 3 + 38

= 41

Sum of n terms ( Sn ) = n/2 [a + an ]

S20 = 20/2 [ 3 + 41 ]

= 10 × 44

= 440

I hope this helps you.

: )

Answer 2

first term = a = 3 

Difference = d = 5-3 = 2 

last term = a + 19d 
               = 3 + 19(2)
               = 3 + 38 
               = 41 

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Then,

sum of n terms = n[first term + last term]/2

sum of 20 terms = 20/2 [first term + last term]

sum of 20 terms = 10 × (3 + 41)
                           = 10 × 44
                           = 440 


i hope this will help you


-by ABHAY