Question

U4 = 16
U9= 51. jumlah 15 suku pertama adalah?

Answer 1

U4 = a + 3b = 16
U9 = a + 8b = 51
semuanya dikurang, jadinya a nya dicorey, 3b - 8b = -5b, 16 - 51 = -35, b = -35/-5 = 7
trus, U4 = a + 3 x 7 = 16, a + 21 = 16, a = 16 - 21 = -5
S15 = 15/2(2 x -5 + 14 x 7)
       = 15/2(-10 + 98)
       = 15/2 x 88 (88 nya dibagi 2 jadinya 44)
       = 15 x 44 = 660

Answer 2

Beda / B = 
        = 7
U4 =16
U3 = 16 -7 = 9
U2 = 9-7 = 2
U1 = 2-7 = -5

U15 = U1 + (n - 1) x B
       = -5 + (15-1) x 7
       = -5 + 14 x 7
       = -5 + 98
       = 93

S(jumlah)15 =  x (U1 + Un (pakai U15))
                  = x (-5 + 93)
                  = x 88
                  =15 x 44
                  =660