Diketauhi barisan aritmatika, jika U3=42, U5+U9=52 maka S20=....???

U3 = a+2b = 42
U5+U9 = a+4b+a+8b = 2a + 12b

a+2b = 42 |x2|
2a+12b = 52 |x1|

2a+4b=84
2a+12b=52
----------------- -
8b = -32
b = 4

2a+12b=52
2a+48=52
2a = 52-48
a = 4 :2 = 2

U20 = a+19b
= 2+19(4)
= 2+76
= 78

maaf klo salah ;D