The included angle of sides JG and GH is angle G (JGH). This is because that is the angle the two line segments create. Angle H is the opposite angle to side JG. If you draw a line from the angle straight down and extended the base, you'd see that the line intersected that side and that side only. The side opposite to angle J is side GH. As I explained above, drawing a line would show you the intersection.
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