Natural Deduction

METHOD OF BACKWARD FELL SWOOP
(based on THE CONJUNCTION OF PREMISES -> CONCLUSION)

Choose the truth values (1 or 0) that make the conclusion false.
Show that this choice makes the conjunction of premises false.

e.g. In the first example, choose D=0. That gives S=1 from the first premise, which gives S =0 from the second premise. So the conjunction of the first and and second premises is 0. hence you have proved the conclusion.