I want to build a seat but can't remember how to figure a isosceles trapezoid by using measurements.
4 Answers

Unless you have a sliding T bevel, don't bother with the angle. Just use direct measurements.
Measure the long and short parallel sides, then the perpendicular distance between the sides (use a square with a ruler or tape measure against it). Also, measure this perpendicular in a few places to be sure they are truly parallel.
Finally, to be sure it is actually isosceles, take a square and straight edge to find the point on the long side directly opposite the outside corner. Then measure the distance from this point to the inside corner. Either transfer this measurement to your work, or repeat at the other side to be sure they are the same.

I take it you want the angle closest to the inside of the room.
Subtract the two parallel sides of the trapezoid and divide by 2. Call this length x.
Measure the vertical distance between the front and back of the intended seat.
The angle = 2nd Function (Tan1) [height / x]
That may be too much of nothing that you want. Maybe you are just trying to cut out the seat.
Start with your rectangle's dimensions. Subtract the bay windows width on the outside from the longest inside dimension.
Divide by 2.
Measure that far in from the part of the seat that will sit next to the othside part of the bay window.

If you know the lengths of the two bases and the depth of the isosceles trapezoid, here is a way to find the acute base angles.
Subtract the smaller base from the larger, then divide this difference by two.
(A picture will really help you understand this) The acute base angle is then
tan^1 (d/x), where d is the depth of the trapezoid and x is the number you calculated above.

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You need to calculate the number of inches of verticle rise for every 12 inches of horizontal run. You need a level horizontal line and a ruller to take these measurements from the side of your building that has the peak on the roof. A carpenter's framing square and a level will work. Many have charts are printed right on them which will help to make the conversions. You can purchase a plastic gauge at your hardware store. Home Depot, Lowes etc. Call a carpenter to assist you.