We see that the regular polygon has 8 vertices ,namely A,B,C,D,E,F,G and H.
So , it also has 8 sides , namely AB,BC,CD,DE,EF,FG,GH,HA.
So we see its an octagon.
We also know that all the sides of a regular polygon are equal and so are their angles.
We know the formula:
Measure of each interior angle of a regular polygon:
 \frac{(2n-4)*90 }{n} degree , where n is the number of sides.
Putting  n here as 8 we get:
45 degree
Since  angle ACH is also an angle of the polygon , angle ACH=45 degree.
Ans:Angle ACH=45 degree.
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