Question

这个matlab程序错在哪儿了，就是不知道啊

>> x=10;
if 4.9<=x&&x<=9.1
y=0.0212432*x^3 - 0.382152*x^2 + 0.706498*x + 22.8;
y=diff(y,x,1)
elseif 9.1<=x&&x<=13.7
y=0.0212432*x^3 - 0.114487*x^2 - 1.37939*x + 20.6;
y=diff(y,x,1)
elseif 13.7<=x&&x<=18.3
y=- 0.0102074*x^3 + 0.178669*x^2 - 1.08415*x + 13.9;
y=diff(y,x,1)
else 18.3<=x&&x<=22.9
y=- 0.0102074*x^3 + 0.0378072*x^2 - 0.0883596*x + 11.7;
y=diff(y,x,1)
end
y
是要得到学x=10时的一阶导函数y的解析式，请高人指点
x=10所在范围的一阶导数解析式

Answer 1

diff是针对符号表达式的，x,y要定义为符号变量，改为下面即可
syms x y;
x1=10
if 4.9<=x1&&x1<=9.1
y=0.0212432*x^3 - 0.382152*x^2 + 0.706498*x + 22.8;
y=diff(y,x,1)
elseif 9.1<=x1&&x1<=13.7
y=0.0212432*x^3 - 0.114487*x^2 - 1.37939*x + 20.6;
y=diff(y,x,1)
elseif 13.7<=x1&&x1<=18.3
y=- 0.0102074*x^3 + 0.178669*x^2 - 1.08415*x + 13.9;
y=diff(y,x,1)
else 18.3<=x1&&x1<=22.9
y=- 0.0102074*x^3 + 0.0378072*x^2 - 0.0883596*x + 11.7;
y=diff(y,x,1)
end
x=x1;
yy=subs(y)

Answer 2

x=10;
syms t
if 4.9<=x && x<9.1
y=0.0212432*t^3 - 0.382152*t^2 + 0.706498*t + 22.8;
elseif 9.1<=x && x<13.7
y=0.0212432*x^3 - 0.114487*t^2 - 1.37939*t + 20.6;
elseif 13.7<=x && x<18.3
y=- 0.0102074*t^3 + 0.178669*t^2 - 1.08415*t + 13.9;
elseif 18.3<= x&& x<22.9
y=- 0.0102074*t^3 + 0.0378072*t^2 - 0.0883596*t + 11.7;
end
y=diff(y)

y =

- (257801805269383*t)/1125899906842624 - 1553055072499647/1125899906842624

将表达式中的x用t代替，因为第一句给x赋值了，往下的y是个固定数值，而不是表达式了，对数值求导自然是0，