11. A school plot of rectangular shape has a perimeter of $160 \mathrm{m}$. How will its area change if the length of each side is increased by 10 m?
11. Let the length of the plot be $x$ m, then its width is $(80-x)$ m (Fig. 19) (the semi-perimeter is $160: 2=80$). If $A D=x$, then $C D=80-x ; C P=10$ m and $E K=$ $=10$ m. The area of the plot will increase by an amount equal to the sum of the areas of the rectangles $A D E M$ and $C E K P$, i.e., by $x \cdot 10+(80-x+10) \cdot 10=10 x+$ $+900-10 x=900$. Thus, the area of the plot will increase by $900 \mathrm{~m}^{2}$.