Tolong di jawab,makasih :)

1.b = 3-1 =2
S49 = 49/2 [2(1) + (49-1) 2]
=>> 49/2 ( 2 + 96)
=>>49/2 (98)
=>> 49 (49)
=>> 2.401

2.1-2+3-4+5-6+..+99-100
=> setiap 2 bilangan hasilnya -1,
jadi jumlahnya adalah
= 100/ 2 (-1)
= 50 (-1)
= -50

3.-100-99-...-0+1+2+..+50
=> -100-99-98-97-...-51
=> S50 = 50/2 [ -100 + (50-1)1]
=> 25 ( -100 + 49)
=> 25 (-51)
=> -1275