tolong kasih tau ini caranya gmn??? yg nomor 5

rumus Sn = n/2 (a+Un) atau n/2 (2a+(n-1)b)
Rumus Un = a + (n-1)b

5a. 1+2+3+....+70
a= 1
b=1
un = 70
sn = 70/2 (1+70)
sn = 35 (71)
sn = 2485

5b. 1+2+3...+100
sn = 100/2 (1+100)
sn = 50 (101)
sn = 5050

5c. 8+9+10+...+85
un = a + (n-1) b
85 = 8 + (n-1) 1
85 = 7 + n
78 = n

sn = 78/2 (8+85)
sn = 3627

5d. 11+12+...+210
210 = 11 + (n-1) 1
210 = 10 + n
200 = n

sn = 100 (11+210)
sn = 22100