去括号简算,要注意变号原式=32.4-12.4+3.8=20+3.8=23.8
32.4+(12.4-3.8)=32.4-12.4+3.8=20+3.8=23.8
32.4-(12.4-3.8)=32.4-12.4+3.8=20+3.8=23.8