Why am i getting 'Array indices must be positive integers or logical values" when trying to plot this function?

x1=linspace(asind(1/1.4),90,100)

% Defining functions for M = 1.4, alpha (a) = 1.4

f1(x1)=2*cotd(x1).*(1.4^2.*(sind(x1)).^2-1)./(1.4^2.*(1.4+cosd(2.*x1))+2)-tand(4)

f2(x1)=2*cotd(x1).*(1.4^2.*(sind(x1)).^2-1)./(1.4^2.*(1.4+cosd(2.*x1))+2)-tand(8)

f3(x1)=2*cotd(x1).*(1.4^2.*(sind(x1)).^2-1)./(1.4^2.*(1.4+cosd(2.*x1))+2)-tand(12)

Answer 1

[Simon Chan]

Remove the x1, otherwise it becomes the index of the varaibles f1,f2 & f3 where index must be a positive integers. That's why you get an error

f1=2*cotd(x1).*(1.4^2.*(sind(x1)).^2-1)./(1.4^2.*(1.4+cosd(2.*x1))+2)-tand(4)
f2=2*cotd(x1).*(1.4^2.*(sind(x1)).^2-1)./(1.4^2.*(1.4+cosd(2.*x1))+2)-tand(8)
f3=2*cotd(x1).*(1.4^2.*(sind(x1)).^2-1)./(1.4^2.*(1.4+cosd(2.*x1))+2)-tand(12)