u10 pada barisan 64,56,48.......adalah.......

a = 64
b = u2-u1 = 56-64 = -8
S10 = n/2 (2a + (n-1)b
= 10/2 (2.64 + 9.-8)
= 5 (128 + (-72)
= 5 (56)
= 280