tentukan jumlah untuk deret deret aritmetika berikut: a) 70+65+60+...+5 b) 15+12+9+...-36

a) a= 70
b= -5

Un= a+(n-1)b
5 = 70+(n-1)-5
5= 70-5n+5
5=75-5n
5-75=-5n
-70=-5n
n=14

Sn=n/2 (a+Un)
= 14/2 (70+5)
= 7 (75)
=525

yang b meyusul dibawah ya

a.
Un = a+(n-1)b
5 = 70+(n-1).-5
5 = 70-5n+5
5 = 70+5-5n
5 = 75-5n
5-75 = -5n
-70 = -5n
n = -70/-5
n = 14

Sn = n/2 (a+Un)
S14 = 14/2 (70+5)
S14 = 7(75)
S14 = 525

b.
Un = a+(n-1)b
-36 = 15+(n-1).-3
-36 = 15-3n+3
-36 = 15+3-3n
-36 = 18-3n
-36-18 = -3n
-54 = -3n
n = -54/-3
n = 18

Sn = n/2 (a+Un)
S18 = 18/2(15-36)
S18 = 9(-21)
S18 = -189