31.4乘0.65加4又2分之1乘3.14减3又50分之7
=31.4×0.65+4.5×3.14-157/50
=31.4×0.65+0.45×31.4-31.4×0.1
=31.4×(0.65+0.45-0.1)
=31.4×1
=31.4