plisss.. jawab ini lehh..

Un = a + ( n - 1 ) b

U3 + U4 = 5
( a + 2b ) + ( a + 3b ) = 5
2a + 5b = 5

U5 + U7 = 40
( a + 4b ) + ( a + 6b ) = 40
2a + 10b = 40

2a + 5b = 5
2a + 10b = 40
---------------------(-)
- 5b = - 35
b = 7

nilai
2a + 5b = 5
2a + 5(7) = 5
2a + 35 = 5
2a = 5 - 35
2a = - 30
a = - 15

Sn = ½ • n ( 2a + ( n - 1 ) b )
S10 = ½ • 10 ( 2(-15) + ( 10 - 1 ) 7 )
= 5 ( - 30 + 63 )
= 5 × 33
= 165 ( B )