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If you have a 360o protractor, you draw one side of the angle, measure 245o round from this line and then draw in the other side of the angle, and then mark a small arc around the reflex angle. If you only have a 180o protractor, you start again by drawing one side of the angle; then you measure 360o - 245o = 115o the "wrong way" round from this line to give the 245o the right way and draw in the other side, and then mark a small arc around the reflex angle.
Take the triangle and label the vertices ABC so that BC is the shortest side.Take your pairs of compasses and set them to the length BC.With the compass point on B draw a small arc to intersect AB and label this point DWith the compass point on C draw a small arc to intersect AC and label this point ENow construct the angle bisectors of BCD and CBE - where these two lines meet is the orthocentre:To construct an angle bisector:Set your compasses at some length.Mark a small arc on each arm of the angle; label the two points X and YWith your compasses set to any length greater than half XY, on one point (say X) draw a small arc between the arms of the angle approximately in the middle - this will be the other side of the point to the vertexWith your compasses on the other point (say Y) draw a small arc to intersect the arc drawn in the last step.Draw in the line between the two vertex and the intersection of the arcs - this is the angle bisector.Note:In constructing the angle bisectors, the first two steps are done in creating points D and E; the points X and Y are:for angle BCD: B and D as you are bisecting the angle at vertex Cfor angle CBE: C and E as you are bisecting the angle at vertex BYou can use your compasses set to the length BC for all the arcs to be drawn.As you have used the length BC to create the points D and E, the triangles created by BCD and CBE are isosceles - in BCD the equal side are BC and BD, in CBE the equal sides are CB and CE. Thus when you draw in the angle bisectors of BCD and CBE they are perpendicular to the base of their respective triangles and thus the heights of those triangles, which is also the height of the original triangle (as point A is on an extension of BD and CE respectively).If there is no one shortest side, either (or both) of points D and/or E will coincide with point A.
A 20 degree angle is classified as an acute angle, which means it measures less than 90 degrees. Acute angles are commonly found in geometric shapes and are known for their sharpness. In trigonometry, a 20 degree angle can be further classified as a specific type of acute angle within the unit circle.
If the angle is greater than that, the pressure increases too quickly and the adverse pressure gradient will cause flow separation along the walls. By keeping the angle small, the flow remains attached and behaves nicely.