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Un = a + (n-1)b
U3 = a + (3-1)b = 18
= a + 2b = 18.... 1
U7 = a + (7-1)b = 38
= a + 6b = 38... 2
a+2b = 18
a+6b = 38
---> -4b = -20
b=5
a+2b = 18
a + 2(5) = 18
a + 10 = 18
a=8
S24 = 24/2 (2x8 + (24-1) 5)
= 12 (16 + 23X5)
= 12 (16 + 115)
= 12x131
= 1572