Question

The 5th and 15 term of ap are 13 and minus 70 respectively find the sum of 21 first term of ap

Answer 1

Hey friend.....
a5 = 13
a15=-70
S21 = ??.
So,
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a+4d =13------->(i)
a+14d = -70--------->(ii)
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From equation (i)
a=13-4d------>(iii)
Now,Putting a= 13-4d in equation (ii)
a+14d= -70
(13-4d)+14d = -70
13 +10d = -70
10d =-70-13
10d = -83
d = -8.3..
Then, Putting d= -8.3 in equ.(iii)
a= 13-4(-8.3)
a= 13 + 33.2
a= 46.2
So , 1st term(a) = 46.2
common difference (d) = -8.3
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S21 = n/2[2a+(n-1)d]
= 21/2 [ 2(46.2) +(20)-8.3)
= 21/2 [ 92.4 + (-166)
= 21/ 2[ -73.6]
= 21(-36.8)
= -772.8....
So,the sum of 21st term of an a.p. is -772.8
HOPE MY ANSWER IS RIGHT☆☆☆☆

Answer 2

here your answer.

hope you understand