13.
(1) solution: let the increased area be x.
The ratio of window area to building area is 10%
∴ The increase rate is10+x/100+x.
∴ 10+x/ 100+x- 10/ 100
=90X/( 1000+ 10X)
∵X & gt; 0
∴90x>; 0 1000+ 10X & gt; 0
∴ 90X/( 1000+ 10X)>0