(a^1/2)-1/(a^1/2)=5^1/2两边平方得a+1/a=7两边再平方得a^2+1/a^2=47
a-1/a={(a-1/a)^2}^1/2={a^2+1/a^2-2}^1/2=45^1/2=3*5^1/2
希望对你能有所帮助.