tentukan hasil dari (tanpa menghitung satu persatu) a.1+3+5+7+9+...+99 b.1-2+3-4+5-6+7-8+...-100 c.-100-99-98-...-2-1-0+1+2+...+48+49+50 tolong ya (pakai cara)

a•
1 + 3 + 5 + ... + 99 ----> 50 suku (n = 50)
= (1 + 99) + (3 + 97) + ... + (49 + 51)
= 100 × 50/2
= 2500

b•
(1 - 2 + (3 - 4) + ... + (99 - 100) ---> n = 100
= (-1) × 100/2
= -50

c•
-100 - 99 - ... - 50 - 49 ... - 0 + 1 + ... + 49 + 50
= -100 - 99 - 98 - ... - 51 + 0
= -(100 + 99 + 98 + ... + 51) ---> n = 50
= -((100 + 51) + (99 + 52) + ... + (76 + 75))
= -(151 × 50/2)
= -(151 × 25)
= -3775