Question

MATLAB难题：如何把一个复杂的含符号表达式按按一定精度化简。

不是simple，是要按一定精度把分式化为小数。
y =1/2115999668407111*(5070602400912917605986812821504-4477454596699003705844035366321*z1^2/(((z1-19/20+x-3/10*(23509722736682911/9007199254740992-6186769141232345/2251799813685248*x)/(19/20-x)/(1+(23509722736682911/9007199254740992-6186769141232345/2251799813685248*x)^2/(19/20-x)^2)^(1/2))*cos((36573973662147359/9007199254740992-6186769141232345/2251799813685248*x)/z1)+((19/20-x)^2/(23509722736682911/9007199254740992-6186769141232345/2251799813685248*x)+3/10/(1+(23509722736682911/9007199254740992-6186769141232345/2251799813685248*x)^2/(19/20-x)^2)^(1/2))*(36573973662147359/9007199254740992-6186769141232345/2251799813685248*x)/z1)^2+((z1-19/20+x-3/10*(23509722736682911/9007199254740992-6186769141232345/2251799813685248*x)/(19/20-x)/(1+(23509722736682911/9007199254740992-6186769141232345/2251799813685248*x)^2/(19/20-x)^2)^(1/2))*sin((36573973662147359/9007199254740992-6186769141232345/2251799813685248*x)/z1)-((19/20-x)^2/(23509722736682911/9007199254740992-6186769141232345/2251799813685248*x)+3/10/(1+(23509722736682911/9007199254740992-6186769141232345/2251799813685248*x)^2/(19/20-x)^2)^(1/2))*cos((36573973662147359/9007199254740992-6186769141232345/2251799813685248*x)/z1))^2))^(1/2)/z1*(((z1-19/20+x-3/10*(23509722736682911/9007199254740992-6186769141232345/2251799813685248*x)/(19/20-x)/(1+(23509722736682911/9007199254740992-6186769141232345/2251799813685248*x)^2/(19/20-x)^2)^(1/2))*cos((36573973662147359/9007199254740992-6186769141232345/2251799813685248*x)/z1)+((19/20-x)^2/(23509722736682911/9007199254740992-6186769141232345/2251799813685248*x)+3/10/(1+(23509722736682911/9007199254740992-6186769141232345/2251799813685248*x)^2/(19/20-x)^2)^(1/2))*(36573973662147359/9007199254740992-6186769141232345/2251799813685248*x)/z1)^2+((z1-19/20+x-3/10*(23509722736682911/9007199254740992-6186769141232345/2251799813685248*x)/(19/20-x)/(1+(23509722736682911/9007199254740992-6186769141232345/2251799813685248*x)^2/(19/20-x)^2)^(1/2))*sin((36573973662147359/9007199254740992-6186769141232345/2251799813685248*x)/z1)-((19/20-x)^2/(23509722736682911/9007199254740992-6186769141232345/2251799813685248*x)+3/10/(1+(23509722736682911/9007199254740992-6186769141232345/2251799813685248*x)^2/(19/20-x)^2)^(1/2))*cos((36573973662147359/9007199254740992-6186769141232345/2251799813685248*x)/z1))^2)^(1/2)-1639176211415221/4503599627370496+z2/z1*(tan(2115999668407111/2251799813685248*z2/(z2+2-2*x))-1639176211415221/4503599627370496)

Answer 1

>> digits(3)
>> vpa(y)

ans =

.473e-15*(.507e31-.448e31*z1^2/(((z1-.950+x-1.*(.783-.824*x)/(.950-1.*x)/(1.+(2.61-2.75*x)^2/(.950-1.*x)^2)^(1/2))*cos((4.06-2.75*x)/z1)+((.950-1.*x)^2/(2.61-2.75*x)+.300/(1.+(2.61-2.75*x)^2/(.950-1.*x)^2)^(1/2))*(4.06-2.75*x)/z1)^2+((z1-.950+x-1.*(.783-.824*x)/(.950-1.*x)/(1.+(2.61-2.75*x)^2/(.950-1.*x)^2)^(1/2))*sin((4.06-2.75*x)/z1)-1.*((.950-1.*x)^2/(2.61-2.75*x)+.300/(1.+(2.61-2.75*x)^2/(.950-1.*x)^2)^(1/2))*cos((4.06-2.75*x)/z1))^2))^(1/2)/z1*(((z1-.950+x-1.*(.783-.824*x)/(.950-1.*x)/(1.+(2.61-2.75*x)^2/(.950-1.*x)^2)^(1/2))*cos((4.06-2.75*x)/z1)+((.950-1.*x)^2/(2.61-2.75*x)+.300/(1.+(2.61-2.75*x)^2/(.950-1.*x)^2)^(1/2))*(4.06-2.75*x)/z1)^2+((z1-.950+x-1.*(.783-.824*x)/(.950-1.*x)/(1.+(2.61-2.75*x)^2/(.950-1.*x)^2)^(1/2))*sin((4.06-2.75*x)/z1)-1.*((.950-1.*x)^2/(2.61-2.75*x)+.300/(1.+(2.61-2.75*x)^2/(.950-1.*x)^2)^(1/2))*cos((4.06-2.75*x)/z1))^2)^(1/2)-.364+z2/z1*(tan(.940*z2/(z2+2.-2.*x))-.364)