Question

Deret Aritmatika
U5 = 11
U8 + U12 = 52

tentukan Sn jumlah 20 suku pertama....

terimakasih:) mohon dibantuuu

Answer 1

U8+U12=a+7b+a+11b=2a+18b=52 ---->a+9b=26
U5=a+4b=11

a+9b=26
a+4b=11 - -----> 5b = 15--->b=3 --->a=11-4b=11-4(3)=11-12=-1

S20=20/2{2(-1)+(19.3)
S20=10{-2+57}
S20=10.55 = 550