Find sum of 20 terms of an ap in which 3rd term is 7 and 7th term is two more than thrice of its 3rd term.
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Answer:sum of 20 terms is 740
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Step-by-step explanation:
Third term=7
Using a+(n-1)d=nth term
a+(3-1)d =7
a+2d=7
Given,
a+ 6d= 2+ 3(a+2d)
a+6d= 2+ 21=23. (substituting a+2d=7)
a+2d =7. -
a+6d. =23
________-
- 4d=-16
d=4
Substitute value of d
a+2d=7
a+(2)(4)=7
a=7-8= - 1
Sum of 20terms=
n/2(2a+(n-1)d)
=20/2(-2+(19)4)
10(-2+76)
=10(74)
=740
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