The nth term of an A.P. is given by ( -4n + 15 ) . Find the sum of first 20 terms of this A.P.
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given, an=-4n+15
therefore, for a1, n=1
a1=-4(1)+15=11
a1=11
now, the last term is the 20th term
therefore,n=20
a20=-4(20)+15=-65
a20=-65
sn=n/2(a+l)
s20=20/2(11+(-65))
s20=10×(11-65)
s20=-540
therefore, the sum is -540
therefore, for a1, n=1
a1=-4(1)+15=11
a1=11
now, the last term is the 20th term
therefore,n=20
a20=-4(20)+15=-65
a20=-65
sn=n/2(a+l)
s20=20/2(11+(-65))
s20=10×(11-65)
s20=-540
therefore, the sum is -540
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