A.l+2+3+4+…20=b. 2+4 +6+8+10…+100=
a. a = 1
b = 2-1
Un = a + (n-1)b
20 = 1 + (n-1) 1
20 = 1 + n – 1
20 = n
maka,
Sn = n/2 (a+Un)
S20 = 20/2 ( 1+20)
S20 = 10(21) = 210
b. a = 2
b = 4-2 = 2
Un = a + (n-1)b
100 = 2 + (n-1) 2
100 = 2 + 2n -2
100 = 2n
n= 50
Sn = n/2 (a+Un)
S50 = 50/2 ( 2 + U50)
S 50 = 25 (2+100) = 25 (102) = 2.550