若x-2y=7,x+2y=5.则x^2-4y^2的值是.另有一题．已知x+1/x=3,则x^2+1/x^2= .
问题描述：
若x-2y=7,x+2y=5.则x^2-4y^2的值是.另有一题．已知x+1/x=3,则x^2+1/x^2= .
答
(1)x^2-4y^2
=x^2-(2y)^2
=(x+2y)(x-2y)
∵x-2y=7,x+2y=5
=5*7=35
(2)x^2+1/x^2=x^2+1/x^2+2*x*1/x-2
=(x+1/x)^2-2
∵x+1/x=3
=3^2-2=7