For each of the following, either construct a formal proof of validity or prove invalidity by means of the STTT. In each case, use the notation in parentheses.

If I buy a new car this spring or have my old car fixed, then I’ll get up to Canada this summer and stop off in Duluth. I’ll visit my parents if I stop off in Duluth. If I visit my parents, they’ll insist on my spending the summer with them. If they insist on my spending the summer with them, I’ll be there till autumn. But if I stay there till autumn, then I won’t get to Canada after all! So I won’t have my  old car fixed. ( N, F, C, D, V, I, A )