1+2+3+4+5+......+80=.....?????

==Jawaban==
a = 1
b = 2 - 1 = 1

Cari "n" nya dulu yaa,,
Un = a + (n - 1)b
80 = 1 + (n - 1)1
80 = 1 + n - 1
n = 80

Jadi,,,,,,,
Sn = n/2(2a + (n - 1)b)
S80 = 80/2(2.1 + (80 - 1)1)
S80 = 40(2 + 79)
S80 = 40(81)
S80 = 3240