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S30 = 15(2a+29d) = 30a + 435d
S20 = 10(2a+19d) = 20a + 190d
30a+435d - 20a - 190d = 755
10a+245d = 755
2a+49d = 151
2(17-5d) + 49d = 151
34 - 10d + 49d = 151
d = 3
a = 17-15 = 2
(b)
S20 = 40+570 = 610
S9 = 4.5(4+8x3) = 126
sum from 10th term to 20th term
= 610 - 126
= 484