syms x1; 
syms x2; 
syms x3; 
syms x4; 
syms x5; 
syms x6; 
syms x7; 
syms y; 
f(1)=int(y/x7*exp(-y^2/2), y, x7, inf)-x1; 
f(2)=(x2-x1)*exp(-x1^2/2)-x1; 
f(3)=(x3-x2)*exp(-x2^2/2)-x1; 
f(4)=(x4-x3)*exp(-x3^2/2)-x1; 
f(5)=(x5-x4)*exp(-x4^2/2)-x1; 
f(6)=(x6-x5)*exp(-x5^2/2)-x1; 
f(7)=(x7-x6)*exp(-x6^2/2)-x1; 
X=(solve(f)); 
