for an AP the 5th term is 30 and the 12th term is 100. find the sum of first 20 terms of the AP.
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Answered by
4
5th term is 30.
Let first term be a
common difference be d.
a+(n-1)d for any nth term.
so:
a+(5-1)d=30
a+4d=30...............(1)
12th term is 100
a+(12-1)d=100
a+11d=100.....................(2)
Subtracting (1) and (2) we get:-
4d-11d=30-100
-7d=-70
d=-70/-7
=10
a+4d=30
a+40=30
a=-10
S=n/2*[2a+(n-1)d]
=20/2*[-20+19*10]
=10*[190-20]
=10*170
=1700.
Hope it helps you.
Answered by
0
Answer:
1700
Step-by-step explanation:
5th term is 30.
Let first term be a
common difference be d.
a+(n-1)d for any nth term.
so:
a+(5-1)d=30
a+4d=30...............(1)
12th term is 100
a+(12-1)d=100
a+11d=100.....................(2)
Subtracting (1) and (2) we get:-
4d-11d=30-100
-7d=-70
d=-70/-7
=10
a+4d=30
a+40=30
a=-10
S=n/2*[2a+(n-1)d]
=20/2*[-20+19*10]
=10*[190-20]
=10*170
=1700
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