(4) (15 pts) Consider the following arguments. If an argument is valid, then present a proof sequence; otherwise, prove that the argument is invalid. You are forbidden to use truth tables to justify your answers (but, you may use them otherwise). (4a) ((p +r) V (q + r) + ((p Vq) + r) (4b) ((1+r)^ (p + (q V r))) + (p+r) (4c) ((7 + (q^r)) ^ (3+) ^ (s + 7) + (t + p)