SI. 1 If $a=-1+2-3+4-5+6-\ldots+100$, find $a$.
SI. 2 The sum of the first $b$ positive odd numbers is $2 a$. Find $b$.
SI. 3 A bag contains $b$ white balls and 3 black balls. Two balls are drawn from the bag at random.
If the probability of getting 2 balls of different colours is $\frac{c}{13}$, find $c$.

SI. 4 If the lines $c x+10 y=4$ and $d x-y=5$ are perpendicular to each other, find $d$.