suku pertama barisan geometri 2 dan suku ketiga adalah 18 untuk r >1 maka jumlah lima suku pertama adalah

Diket :
U(1) = 2
U(3) = 18
r > 1

Jawab :
U(1) = a.r^0 = 2
U(1) = a.1 = 2
U(1) = a = 2

U(3) = 2.r^2 = 18
U(3) = r = √18/2
U(3) = r = √9
U(3) = r = 3

Sn = a(r^n-1)/r-1
S(5) = 2(3

U1=2
U3=18
r=U3/U1=ar^2/a=r^2=18/2=9
r^2=9

[tex] \sqrt{9} [/tex]
r=3

U1=a=2
S5= a(r^5-1)/r-1
= 2(3^5 -1)/3-1
= 2(243 - 1)/2
=2(242/2)
=242