Tolong Bantu jawab ya

U2= 11 =>a+b=11----1)
U4= 19 =>a+3b=19---2)
Pers.2)&1)
a+3b=19
a+b = 11
------------ -
2b=8
b=4
Pers. 1)
a+b =11
a+4 =11
a =7
Maka S5=5/2(2a+4b)
=5/2(2•7+4•4)
=5/2(14+16)
=5/2(30)
=75