4. The common difference of an AP is -3.Find its sum, if its first term is
75 and the last term is 33.
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d = -3 ; a = 75 ; a(n) = 33
♥ n - 1 = ( a(n) - a ) / d
= ( 33 - 75 ) / -3
= 42 / 3
= 14
n = 14 + 1 ➡ n = 15
♥ S(n) = n/2 ( a + a(n) )
= 15/2 ( 33 + 75 )
= 15/2 x 108
= 15 x 54
S(n) = 810
♥ n - 1 = ( a(n) - a ) / d
= ( 33 - 75 ) / -3
= 42 / 3
= 14
n = 14 + 1 ➡ n = 15
♥ S(n) = n/2 ( a + a(n) )
= 15/2 ( 33 + 75 )
= 15/2 x 108
= 15 x 54
S(n) = 810
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