Apa maksud no 3? Tolong jawab saya lagi perlu bukan PR

Barisan dan Deret
IX SMP

a. 1 + 3 + 5 +7 + ..... +99

U1 = 1
b = 2
Un = 99

Un = U1 + (n-1) b
99 = 1 + (n -1) 2
99 = 1 + 2n - 2
99 = 2n - 1
99 + 1 = 2n
100 = 2n
50 = n

Sn = n/2 ( 2.U1 + (n -1) b)
S50 = 50/2 ( 2.1 + ( 50 -1) 2)
S50 = 25 ( 2 + ( 49) 2)
S50 = 25 ( 2 + 98 )
S50 = 25 ( 100 )
S50 = 2500

c. -100 - 99 - 98 - .... - 2 ... + 50

U1 = -100
b = 1
Un = 50

Un = U1 + (n -1) b
50 = -100 + ( n - 1) 1
50 = -100 + n - 1
50 = n - 101
50 + 101 = n
151 = n

Sn = n/2 ( 2.U1 + (n-1) b)
S151 = 151/2 ( 2(-100) + (151 - 1) 1)
S151 = 75,5 ( -200 + 150 )
S151 = 75,5 ( -50)
S151 = -1510 => Kenapa minus ? karena deretan minus lebih banyak daripada deetan positifnya