0.78x0.78=78x78÷10000=(80-2)X(80-2)÷10000=[80x80-2x80x2+2x2]÷10000=(6400-320+4)÷10000=6084÷10000=0.6084或者0.78x0.78=(0.8-0.02)X(0.8-0.02)=[0.8x0.8-2x0.8x0.02+0.02x0.02]=(0.64-0.032+0.0004)=0.6084