a²+1/a²=47a²-2+1/a²=47-2(a-1/a)²=45所以有：a-1/a=±√45=±3√5
(a-1/a)^2=a^2+1/a^2-2(a-1/a)^2=47-2=45a-1/a=±3√5
(a-1/a)^2=a^2+1/a^2-2=45a-1/a=3根号5 或-3根号5
a-1/a=√(a-1/a)²=√(a²+1/a²-2)=√(47-2)=3√5
