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
2 answers:
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
400 is right ^^^^^ it should be
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