Because v1 x v2 + v2 x (-v1) = v1 x v2 - v2 x v1 = v1 x v2 - v1 x v2 = 0, vector (v1,v2) is orthogonal to (v2,-v1);
Let (a,b) be an unit vector orthogonal to (9,40);
So, a^2 + b^2 = 1 and a x 9 + b x 40 = 0;
Then, a = - 40 x b / 9;
Finally, 160 x b^2 / 81 + b^2 = 1;
160 x b^2 + 81 x b^2 = 81;
241 x b^2 = 81;
b^2 = 81 / 241;
b = + or - 9/
a = + or - 40/
We have (+40/;9/) and (-40/;-9/);
Let (a,b) be an unit vector orthogonal to (9,40);
So, a^2 + b^2 = 1 and a x 9 + b x 40 = 0;
Then, a = - 40 x b / 9;
Finally, 160 x b^2 / 81 + b^2 = 1;
160 x b^2 + 81 x b^2 = 81;
241 x b^2 = 81;
b^2 = 81 / 241;
b = + or - 9/
a = + or - 40/
We have (+40/