Hitunglah JUMlah bilangan berikut :
a.) 3 + 6 + 9 + 12 + ... + 42.
b.) ( -12) + ( -7) + ( -2) + ... + 78
c.) ( -2) + 5 + 12 + ... + 145
a.) 3 + 6 + 9 + 12 + ... + 42.
b.) ( -12) + ( -7) + ( -2) + ... + 78
c.) ( -2) + 5 + 12 + ... + 145
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145 = (-2) + (n-1)7
145 = -2 + 7n - 7
145 = -9 +7n
145 + 9 =7n
154 = 7n
n = 154/7
n = 22
b=7
Sn = n/2 (2a + (n-1)b)
Sn = 22/2 (2(-2) + (22-1)7)
Sn = 11(-4 + 147)
Sn = 11(143)
Sn = 1573