+60 \times 100-40 \times 40 6 \times 6=36(cm^{2}) 4 \times 4=16(cm^{2}) \left{ \begin{matrix} 160 \times 100-40 \times 40-(160+40)\times(100-40)\div 2 \\ =1600-1600-6000 \\ =14400-6000 \\ =8400(cm^{2}) \end{matrix} \right. 36+16=52(cm^{2}) ...