Question

Straight angles Are extremely important in geometry. When two lines intersect , they form multiple angles. In the diagram below, line gm and fh intersect at e. If any of the four angles formed is given, all other three can be found quickly. Fill in the table below.

Answer 1

When two straight lines intersect, the vertical opposite angles intersect. the other two angles are also equal. Let the known angle be x, then the other two adjacent angles are obtained subtracting twice of x from 360 and dividing the result by 2.

Therefore, the table can by filled as follows:

Row 1:

Given <GEF = 120°

<FEM is adjacent to <GEF, thus
$\angle FEM= \frac{360-2(120)}{2} \\ \\ = \frac{360-240}{2} = \frac{120}{2} =60^o$

<MEH is vertically opposite to <GEF and thus is equal to <GEF. Thus <MEH = 120°

<HEG is vertically opposite to <FEM and thus is equal to <FEM. Thus <HEG = 60°.



Row 2:

Given <MEH = 150°

<MEH is vertically opposite to <GEF and thus is equal to <GEF. Thus <GEF = 150°

<FEM is adjacent to <GEF, thus
$\angle GEF= \frac{360-2(25)}{2} \\ \\ = \frac{360-50}{2} = \frac{310}{2} =155^o$

<HEG is vertically opposite to <FEM and thus is equal to <FEM. Thus <HEG = 30°.



Row 3:

Given that <FEM = 25°

<FEM is adjacent to <GEF, thus
$\angle GEF= \frac{360-2(25)}{2} \\ \\ = \frac{360-50}{2} = \frac{310}{2} =155^o$

<MEH is vertically opposite to <GEF and thus is equal to <GEF. Thus <GEF = 155°

<HEG is vertically opposite to <FEM and thus is equal to <FEM. Thus <HEG = 25°.



Row 4:

Given that <HEG = 45°

<HEG is adjacent to <GEF, thus
$\angle GEF= \frac{360-2(45)}{2} \\ \\ = \frac{360-90}{2} = \frac{270}{2} =135^o$

<HEG is vertically opposite to <FEM and thus is equal to <FEM. Thus <FEM = 45°

<MEH is vertically opposite to <GEF and thus is equal to <GEF. Thus <GEF = 135°.

Answer 2

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