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Ivenika [448]
3 years ago
5

The Sum of first seven terms of an AP is 49 and the sum of first seventeen terms is 289 find the sum of first 20 terms​

Spanish
2 answers:
Afina-wow [57]3 years ago
6 0

Answer:

S7=49

7/2(2a+6d)=49

2a+6d=49×2/7

2a+6d=14

2a=14-6d

a=7-3d........1

S17=289

17/2×(2a+16d)=289

2a+16d=289×2/17

2a+16d=34

a+8d=17

substituting value of a

7-3d+8d=17

5d=17-7

d=10/5=2

substituting value of d in equation 1

a=7-3×2=1

<h3>now</h3>

S20=20/2(2a+19d)=10(2×1+19×2)=10(40)

=40×10=400

zaharov [31]3 years ago
6 0
400 is right ^^^^^ it should be
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