how to find cosine angle known the end points of 2 linesegments

There are 2 line segments, known their end points, how can i find angle between them

[Guillaume]

This is basic math, that a simple web search would answer straight away

Note: you calculate the dot product of two vectors with dot and the norm with norm.

Answer 1

[Guillaume]

The arguments you pass to dot and norm must be vectors not scalars

v1 = [1 1]
v2 = [0 3]
D = [v1;v2]; %possibly you have something like this

costheta = dot(D(1, :), D(2, :)) / (norm(D(1, :)) * norm(D(2, :)));
thetaindegrees = acosd(costheta)

Answer 2

[Roger Stafford]

Using 'atan2' or 'atan2d' and cross product is more accurate for angles that are near zero or near pi (180 degrees.)

   a = atan2(norm(cross(D1,D2)),dot(D1,D2));  % Angle in radians

or

   a = atan2d(norm(cross(D1,D2)),dot(D1,D2));  % Angle in degrees

[krishnasri]

I've around 60 such values in excel sheet and I need to calculate angle between all these points. How can I put the same in a loop for calculating it for 60 values?

D = xlsread('45_A.xls');

for i=1:60

CosTheta = dot(D(i,:),D(i+1,:))/(norm(D(i,:))*norm(D(i+1,:)));

ThetaInDegrees = acosd(CosTheta);

end

xslwrite('result',ThetaInDegrees);

but there is some error in the values. can u please help me