Tolong dikerjakan ??

31 )U10 - U5 = 10
a + 9b - ( a + 4b ) = 10
a + 9b -  a - 4b  = 10
5b = 10
b = 10/5
b = 2
U3 + U8 = 24
a + 2b + a + 7b = 24
2a + 9b = 24
2a + 9*2 = 24
2a + 18 = 24
2a = 24 - 18
2a = 6
a = 6 : 2 = 3
S40 = n/2{2a+(n-1)*b}
=40/2{2*3+39*2}
= 20 {6+78}
=20*84
=1680