karthiknayal2 karthiknayal2

25-06-2020
Mathematics

contestada

find the sum of 1st 20 terms of an AP in which third term is 7and 7th term is 2more than thrice of its 3rd term

Respuesta :

dannielle dannielle

25-06-2020

Answer:

S=740

Step-by-step explanation:

a3=7

a7=3*a3+2=3*7+2=23

a7=a3+4r

23=7+4r

-7 -7

16=4r

r=16/4

r=4

a20=a7+13r

a20=23+13*4

a20=23+52

a20=75

a1=a3-2r

a1=7-2*4

a1=7-8

a1=-1

S=(a1+a20)*20/2=(-1+75)*10=74*10=740

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