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

Question

Mathematics

Marisa

16 March, 05:53

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

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Answers (1)

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Rhys Deleon · Answer 1

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