Write the required term in each of these patterns: a) In the number pattern 1; 4; 9; ..., find the fifth term. b) In the number pattern 2000; 200; 20; ..., find the fourth term. c) In the number patte

$$
20 \div 10 = 2
$$
The fourth term is $\boxed{\text{2}}$.

The given terms are: $T_1 = 10\frac{1}{2}$, $T_2 = 10$, $T_3 = 9\frac{1}{2}$, $T_4 = 9$.  
$T_5 = 9 - \frac{1}{2} = 8\frac{1}{2}$  
$T_6 = 8\frac{1}{2} - \frac{1}{2} = 8$  
The sixth term is $\boxed{\text{8}}$.

$2 - 1 = 1$  
$4 - 2 = 2$  
$7 - 4 = 3$  
The differences are increasing by 1 each time.

The next difference will be 4, so $T_5 = 7 + 4 = 11$.  
The next difference will be 5, so $T_6 = 11 + 5 = 16$.  
The sixth term is $\boxed{\text{16}}$.

The given terms are: $T_1 = 1$, $T_2 = 0.9$, $T_3 = 0.8$, $T_4 = 0.7$.  
$T_5 = 0.7 - 0.1 = 0.6$  
$T_6 = 0.6 - 0.1 = 0.5$  
$T_7 = 0.5 - 0.1 = 0.4$  
The seventh term is $\boxed{\text{0.4}}$.