obtain the sum of first 56 terms of an AP whose 18th and 39th term is 52 and 148 respectively
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18th term=a+19d=52..... equation 1
39th term=a+38d=148..... equation 2
from equation 1 and equation 2
a+38d-(a+19d)=148-52
a+38d-a-19d=96
19d=96
d=96/19
from equation 1
a+19d=52
a+19(96/19)=52
a+96=52
a=-44
sum of first 56 terms =n/2 (a+an)
=56/2(-44+(a+(n-1)d))
=28(-44+(-44+(56-1)96/19))
=28(-44+(-44+(57)96/19))
=28(-44+(-44+288))
=28(-44+244)
=28(200)
=5600
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