matlab求函数方程的正根,matlab解方程组的时候，可以只要正根吗？

程序如下：

syms J1 J2 J3 J4

la=375;

lb=785;

R=275;

r=102;

x=100;

y=100;

z=-600;

J=0;

[J1,J2,J3,J4]=solve((1.414*r*sin(0.785-J)*cos(J)+x-R-la*sin(J1))^2+(1.414*r*sin(0.785-J)*sin(J)+y)^2+(z+la*cos(J1))^2-lb^2,...

(-1.414*r*cos(0.785-J)*sin(J)+x)^2+(1.414*r*cos(0.785-J)*cos(J)+y-R-la*sin(J2))^2+(z+la*cos(J2))^2-lb^2,...

(1.414*r*sin(0.785-J)*cos(J)-x-R-la*sin(J3))^2+(1.414*r*sin(0.785-J)*sin(J)-y)^2+(z+la*cos(J3))^2-lb^2,...

(1.414*r*cos(0.785-J)*sin(J)+x)^2+(-1.414*r*cos(0.785-J)*cos(J)+y-R+la*sin(J4))^2+(z+la*cos(J4))^2-lb^2)

J1=vpa(J1)

J2=vpa(J2)

J3=vpa(J3)

J4=vpa(J4)

这时得到的解是：

J1 =

1.6726496034324791422369776889755

-1.9149768196177261917706095593390

1.6726496034324791422369776889755

-1.9149768196177261917706095593390

1.6726496034324791422369776889755

-1.9149768196177261917706095593390

1.6726496034324791422369776889755

-1.9149768196177261917706095593390

J2 =

1.6728134907996934474665869127185

-1.9148739472216682383904294004894

1.6728134907996934474665869127185

-1.9148739472216682383904294004894

J3 =

1.2065390806953418416441018033182

-2.0607020564450271187956692450830

1.2065390806953418416441018033182

-2.0607020564450271187956692450830

1.2065390806953418416441018033182

-2.0607020564450271187956692450830

1.2065390806953418416441018033182

-2.0607020564450271187956692450830

1.2065390806953418416441018033182

-2.0607020564450271187956692450830

1.2065390806953418416441018033182

-2.0607020564450271187956692450830

1.2065390806953418416441018033182

-2.0607020564450271187956692450830

1.2065390806953418416441018033182

-2.0607020564450271187956692450830

J4 =

-1.1965019139970002022096767620101

2.0615980649040293018453204462917

可是我只想要：

J1 =1.6726496034324791422369776889755

J2 =1.6728134907996934474665869127185

J3 =1.2065390806953418416441018033182

J4 =2.0615980649040293018453204462917

有什么好办法吗？各位给点提示吧谢谢了