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Sagot :
Answer:
1,500
Step-by-step explanation:
a + 5d = 39 (1)
a + 18d = 78 (2)
Subtract (1) from (2) to eliminate a
18d - 5d = 78 - 39
13d = 39
d = 39/13
d = 3
Substitute d = 3 into (1)
a + 5d = 39 (1)
a + 5(3) = 39
a + 15 = 39
a = 39 - 15
a = 24
Sum of the first 25 terms
Sn = n/2[2a + (n – 1)d]
S25 = 25/2{2*24 + (25-1)3}
= 12.5{48 + (24)3}
= 12.5{48 + 72)
= 600 + 900
= 1,500
S25 = 1,500
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