求中间的剩余面积
设正方形边长x,
中间剩余部分面积S
=13×7-2[x²+(13-x)(7-x)]
=91-2[2x²-20x+91]
= -4x²+40x-91
=(-2x+13)(2x-7)
当x=4时,S=5 cm²;
当x=5时,S=9cm²;
当x=6时,S=5cm²