5a-(3b的2次方+7ab）+(2b的2次方-5a的2次方)
=5a-3b²-7ab+2b²-5a²
=5a-b²-7ab-5a²
当a=0.1 b=0.2时
原式=0.5-0.04-0.07-0.05
=0.34