Question

Suku ke-4 dan suku ke-7 barisan aritmatika masing-masing 19 dan 37.Jumlah 24 suku pertama adalah...
a.1.752
b.1.740
c.1.680
d.1.668

Answer 1

U7 = a + 6b ⇒ 37 = a + 6b
u4 = a + 3b ⇒ 19 = a + 3b
                        _________  -
                        18 = 3b
                           b = 6
19 = a + 3b
19 = a +18
a = 1

u24 = a + 23b
u24 = 1 + 23(6) = 139

s24 = 24/2 (1+139) = 1680 (c)