已知x-1/x=5,求（1）x²+1/x²；（2）x四次方+1/x四次方（3）x²-5x-7

(1)
x-1/x=5 平方一下
x^2-2+1/x^2=25
x^2+1/x^2=25+2=27
(2)
x^2+1/x^2=27 平方一下
x^4+2+1/x^4=729
x^4+1/x^4=729-2=727
(3)
x-1/x=5
x^2-1=5x
x^2-5x=1
x^2-5x-7
=1-7
=-6