Question

in an A.P 19th term is 52 and 39th term is 128,find of first 56 term​

Answer 1

Step-by-step explanation:

a19 = a+ 18d = 52........... 1

a38 =a+ 37d = 128..........2

- - -

= - 19d =-76

d = - 76/-19

d = 4

Putting this value of d in 1

a + 18 × 4 = 52

a + 72 = 52

a = 52 - 72

a= - 20

So

S56 = n/2 (2a +(n-1)d)

= 56/2 (2×-20 +(56 - 1) 4)

=28 (-40 + 55×4)

=28 (180)

=5040

Hope This will help you

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