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1.
SECTION -
Find the sum of first 20 terms of an AP a which the 3 term is 7 and 7 terms]
more than three of its 3 tem
Show that the manis - 521 and 19 5) are collect
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Answer :- a+2d=7
a+6d=3(a+2d)+2
a+6d=3*7+2
a7 = 21+2
a7 = 23
a+2d=7
a+6d=23
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-4d = -16
d = 4
a = -1
S20 = n/2[2a+(n-1)d]
= 10 [2(-1) +19*4]
= 10 [-2+76]
= 10[74]
= 740
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