p ^ 3+q ^ 3=2
(p+q)(p^2-pq+q^2)=2
p^2-pq+q^2=2/(p+q)
假设p+q>2
(p+q)^2-3qp=pq>1 (2)
(1)(2)互相矛盾,假设不成立,所以p+q≤2