（-42.75）*（-27.36）-（-72.64）*（+42.75）=?后面有个）（+42.75）为什么前面得负请说明原因

原式=(-42.75)×(-27.36)-(-72.64)×(+42.75)
=42.75×27.36-[-(72.64×42.75)]
=42.75×27.36+72.64×42.75
=42.75×27.36+42.75×72.64
=42.75×(27.36+72.64)
=42.75×100
=4275