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U5 + U7 = 36
S10 = ?
U5 + U7 = a + 4b + a + 6b
= 2a + 10b
U3 = a + 2b
U5 + U7 = 2a + 10b = 36
U3 = a + 2b = 9 -
a + 8b = 27
U9 = 27
U9 = a + 8b = 27
U3 = a + 2b = 9 -
6b = 18
b = 18 / 6
b = 3
U3 = a + 2b
9 = a + 2 * 3
9 = a + 6
9 - 6 = a
a = 3
U10 = a + 9b
= 3 + 9 * 3
= 3 + 27
= 30
S10 = n / 2 (a + U10)
= 10 / 2 (3 + 30)
= 5 * 33
= 165