tolong di bantu kak u.u

u3+u5+u7= 45
a+2b+a+4b+a+6b= 45
3a +12 b = 45 ........ sederhanakan bagi 3 =
a + 4b = 15 ....... (1)

u2+u6 = 35
a+b+a+5b= 35
2a+6b = 35 ............(2)

eliminasi pers (1) dan (2)
(1) a + 4b=15 (kalikan 2) => 2a+8b= 15
(2)2a+6b = 35 (kalikan 1) => 2a+6b= 35
------------------ -
2b=-20
b = -10

subtitusi ke pers (1)
a+4(-10) = 35
a -40 = 35
a = 75

a. suku pertama = a = 75
b.selisih = beda = b = -10
c. U16 = a+15b = 75 + 15(-10) = 75-150 = -75
d. S20 = 20/2(2.75 +19(-10)
= 10(150-190)
= 10(-40)
= -400