(i) - 9, -14, - 19, - 24, ......
a = - 9, d = - 14 + 9 = - 5
a30= a + 29d
= - 9 + 29(- 5) = -154
a20 = a + 19d
= - 9 + 19(- 5) = - 104
a30 – a20 = -154 – (-104)
= -154 + 104
= - 50
(ii) a, a + d, a + 2d, a + 3d, ....
a = a, d = a + d – a = d
a30 = a + 29d
a20 = a+ 19d
a30 – a20 = a + 29d – a – 19d
= 10d
