估算21×49时,可以把21看做*~.m.O0x(W9V.l8{0`1o,把49看做8|-X5z(T8Q,|+b']1B:A/V;s)p,积大约是*H0y9]/X)y5r)y5^1Q2f3Y5p+d6c6I。.p8t0t/j1C9R