no 19 Sama 20 Sama cara nya

20). 105,110,115.......295
Maka, a = 105
b = 5
* Un = a+(n-1)b
295 = 105 + 5n - 5
5n = 195
n = 39
》Sn = 1/2 n (a+Un)
= 39/2 (105 + 295)
= 39/2 (400)
Sn = 7800