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Question
In an A.P., if the 5th and 12th terms are 30 and 65 respecitvely, what is the sum of first 20 terms?
In an A.P., if the 5th and 12th terms are 30 and 65 respecitvely, what is the sum of first 20 terms?
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Solution
a+4d=30
a+11d=65
a+11d - a-4d=35
7d=35
d=5
a+20=30
a=10
s20 = n/2(2a+(n-1)d)
=20/2(20 + 95)
= 10(115)
= 1150
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a+4d=30
a+11d=65
a+11d - a-4d=35
7d=35
d=5
a+20=30
a=10
s20 = n/2(2a+(n-1)d)
=20/2(20 + 95)
= 10(115)
= 1150
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