4^3=64 76^(1/3)=(64+12)^3=4(1+3/16)^(1/3)
设f(x)=x^(1/3) f'(x)=(1/3)/x^(2/3)
f(76)=f(64+12)=f(64)+△y
≈4+dy= 4+12f'(4)=4+4/16=4.25