Rumus deret 1. 98,89,80,71........410 2. - 40,-32,-24..............420 3.-11,-18,-25...........421 Poin nya banyak looh

1. b = 89-98 = -9
Sn = n/2 [2(98) + (n-1)-9]
= n/2 ( 196 + 9 - n)
= n/2 ( 205 -n)

2.b = -32-(-40)= 8
Sn = n/2 [ 2(-40 + (n-1)8)]
= n/2 ( -80 +n -8)
= n/2 ( -88 +n)

3.b = -18-(-11) = -7
Sn = n/2 ( 2(-11) + (n-1)(-7) )
= n/2 ( -22+7-n)
= n/2 ( -15 -n)