Please repeat the question of the exercise sheet so that also students of later semesters can understand what we are discussing here. The question is that for a given LTL formula φ, we should construct two LTL formulas φ1 and φ2 that satisfy the following requirements:
- φ1 and φ2 are not equivalent
- φ1 and φ2 are both satisfiable
- φ1->!φ and φ2->!φ are valid
For the given formula φ=[[c SU b] WU a], you suggest φ1=G(!a & b & !c) and φ2=G(!a & b & !c) which does obviously violate the first requirement: The formulas are even syntactically the same, so they are definitely equivalent.
For the second formula φ=[c SB [a SB b]], you suggest φ1=G(a & !b & !c) and φ2=G(!a & !b & c) which are satisfiable and not equivalent. Also, φ1->!φ is valid, but φ2->!φ is not valid: A counterexample is a single path where always !a&!b&c holds which obiviously satisfies φ2 but also satisfies φ.