Question

Suatu barisan aritmatika , U10 = (-x+40) , U11 = (-3x+16) sedangkan U20 = (-8x-5) dan U21 = (-9x-14) , jadi berapakah U40+ U50 = …..

Answer 1

U10 = (-x+40) = a + 9b
U11 = (-3x+16)= a + 10b
------------------------------ ( - )
selisih = 2x + 24 = -b .. pers 1
U20=(-8x-5)=a+19b
U21=(-9x-14)=a+20b
-------------------------- (-)
x + 9 = -b .... pers 2
pers 1 dan 2 di eliminasi
2x + 24 = -b
x + 9 = -b
--------------- (-)
x + 15 = 0
x = -15
x + 9 = -b
-15 + 9 = -b
-6 = -b
b = 6
U21 = -9x-14=a+20b
-9(-15)-14=a+20(6)
135-14=a+120
121= a + 120
a = 121 - 120 = 1
U40= a+39b=1+(39x6)=235
U50=a+49b=1+(49×6)=295
---------------------------------- (+)
jadi U40 + U50 = 530