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