Question

matlab中求极值点

t=0:1/2500:0.1

x=sin(t).*cos(2*t)
提取x的极值点
x=sin(2*pi*f1*t).*cos(2*pi*f2*t)
f1=60
f2=60*37

Answer 1

先画出导数的曲线,根据其零点判断有几个极点,然后对各极点分别指定适当的初值,利用fsolve求出准确的零点位置,再绘图即可.参考代码如下：symsxf=-2*x.^2+4+sin(2*pi*x);df=diff(f);ezplot(df,[-33])x0=[-1.8-1.3-0.80.50.20.51.2];x0=fsolve(inline(char(df)),x0,optimset('fsolve'));holdonplot(xlim,[00],'--c')plot(x0,subs(df,x0),'ro')figure(2)ezplot(f,[-33])holdonplot(x0,subs(f,x0),'ro')

Answer 2

可这样：
t=0:1/2500:0.1;x=sin(t).*cos(2*t);
f1=60;f2=60*37;x1=sin(2*pi*f1*t).*cos(2*pi*f2*t);
plot(t,x,t,x1)

a=max(x),b=min(x)%最大、最小值
a1=max(x1),b1=min(x1)%最大、最小值

Answer 3

data是你的数据，
find(diff(sign(diff(data)))==-2)+1
找到极大值的位置

find(diff(sign(diff(data)))==2)+1
找到极小值的位置

Answer 4

function hh
global dy1 dy2
y='x^2*sin(x^2-x-2)'
dy1=diff(y)
dy2=diff(y,2)
subplot(3,1,1)
ezplot(y,[-2 2])
subplot(3,1,2)
ezplot(dy1,[-2 2]),hold on,plot(-2:2,zeros(length(-2:2)))
subplot(3,1,3)
ezplot(dy2,[-2 2]),hold on,plot(-2:2,zeros(length(-2:2)))
x01=fsolve(@myfun1,[-1.5 -0.7 0 1.6])
x02=fsolve(@myfun2,[-1.9 -1.3 -0.5 1.3])
function f1=myfun1(x)
global dy1
f1=subs(dy1);%very inportamt!!!!!;
function f2=myfun2(x)
global dy2
f2=subs(dy2);%very inportamt!!!!!;

结果：
y =

x^2*sin(x^2-x-2)

dy1 =

2*x*sin(x^2-x-2)+x^2*cos(x^2-x-2)*(2*x-1)

dy2 =

2*sin(x^2-x-2)+4*x*cos(x^2-x-2)*(2*x-1)-x^2*sin(x^2-x-2)*(2*x-1)^2+2*x^2*cos(x^2-x-2)

Optimization terminated: first-order optimality is less than options.TolFun.

x01 =

-1.5326 -0.7315 0 1.5951

Optimization terminated: first-order optimality is less than options.TolFun.

x02 =

-1.9240 -1.2650 -0.4742 1.2404

Answer 5

data是你的数据，
find(diff(sign(diff(data)))==-2)+1
找到极大值的位置

find(diff(sign(diff(data)))==2)+1
找到极小值的位置