一、口算。
$0.05×40=$ $8.4÷20=$ $4.9+(8.7+5.1)=$
$5.4-1.04=$ $0.38×1000=$ $1.2×5÷1.2=$
$x+x=$ $0.12+0.8=$ $26.1-3.5-7.5=$
$12÷1.2=$ $0.44÷0.2=$ $(2.5+0.25)×4=$
$0.25×1.6=$ $6.18÷0.6=$ $3.2÷0.1÷0.01=$
$0.34×5=$ $3.08-2.8=$ $(1-0.01)×100=$
$6.84÷0.6=$ $1.8×40=$ $0.99+0.01÷10=$
$6.3÷0.3=$ $4.5a÷9=$ $400×7.5×2.5=$

2 0.42 18.7 4.36 380 5 2x 0.92 15.1 10 2.2 11 0.4 10.3 3200 1.7 0.28 99 11.4 72 0.991 21 0.5a 7500

$0.05×40=2$
$8.4÷20=0.42$
$4.9+(8.7+5.1)=18.7$
$5.4-1.04=4.36$
$0.38×1000=380$
$1.2×5÷1.2=5$
$x+x=2x$
$0.12+0.8=0.92$
$26.1-3.5-7.5=15.1$
$12÷1.2=10$
$0.44÷0.2=2.2$
$(2.5+0.25)×4=11$
$0.25×1.6=0.4$
$6.18÷0.6=10.3$
$3.2÷0.1÷0.01=3200$
$0.34×5=1.7$
$3.08-2.8=0.28$
$(1-0.01)×100=99$
$6.84÷0.6=11.4$
$1.8×40=72$
$0.99+0.01÷10=0.991$
$6.3÷0.3=21$
$4.5a÷9=0.5a$
$400×7.5×2.5=7500$

二、计算下面图形的面积。

$18×18÷2=162(cm^{2})$ $9÷2+3-1=6.5(cm^{2})$

1. $18×18÷2 = 162\ \text{cm}^2$
2. $9÷2 + 3 - 1 = 6.5\ \text{cm}^2$