Fill in the blank. \text { (a) } \bigcup_{i=1}^{n}\{1-i, 2+i\}=\ldots \ldots \ldots \ldots (b) Suppose b is any integer. If b mod 12 = 5, then 7b mod 12 =

\text { (c) Compute the summation. } \sum_{k=1}^{5}(-1)^{k}\left(2^{k}+3\right)=\ldots \ldots \ldots \ldots (d) The negation for the statement "Vn E Z, if n is divisible by 21, then n is divisibleby 3 and n is divisible by 7" is (e) The contrapositive for the statement "a is even if a is divisible by 6" is