Find the sum of the first 20 terms of an arithmetic progression, if its fifth term is 11 and its 16th term is 39.
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given 5th term is 11.
a+ 4d= 11 (equation 1). also given 16 th term is 39. which implies
a+ 15 d =39.(equation 2)
now (equation 2) - (equation 1) implies
39-11= (a-a)+ (15d - 4d)
28=11d
d= 28/11= 2.5454
hence from equation 1 we get
a+ (4*2.5454)=11
implies a= 0.8181
now S20= 20/2(2*0.8181+(19*2.5454))
hence sum of first 20 terms= 499.988
a+ 4d= 11 (equation 1). also given 16 th term is 39. which implies
a+ 15 d =39.(equation 2)
now (equation 2) - (equation 1) implies
39-11= (a-a)+ (15d - 4d)
28=11d
d= 28/11= 2.5454
hence from equation 1 we get
a+ (4*2.5454)=11
implies a= 0.8181
now S20= 20/2(2*0.8181+(19*2.5454))
hence sum of first 20 terms= 499.988
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