In the figure, a piece of paper PQRS in the shape of a parallelogram is folded along 2 dotted lines, TS and UR as shown in Figure 1. In Figure 2, and .

- (a)
Find in Figure 2.
- (b)
Find in Figure 2.
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Worked solution
10(a)
1. Relate the folded angle $\angle UQR$ to the original angle of the parallelogram.
When the paper is folded, the angle in Figure 2 corresponds directly to the original interior angle of the parallelogram PQRS before folding because the shape around Q is preserved under folding.
Since PQRS is a parallelogram, the opposite interior angles are equal:
2. Calculate the original interior angle $\angle PSR$ to find $\angle UQR$.
The fold line is perpendicular to the parallel horizontal sides and of the parallelogram. Therefore, the angle between the vertical line and the base is (i.e., ).
The full angle at before folding, , is the sum of the angle of the folded part and the right angle:
Thus,
10(b)
1. Find the interior angles of the original parallelogram PQRS.
In the original parallelogram , we are given in the folding diagram that after folding along the vertical line , the remaining angle is .
Since , in the unfolded state, the angle at of the right-angled triangle is .
Therefore, the original angle of the parallelogram is:
Since adjacent angles in a parallelogram sum to , the original angle is:
2. Use the symmetry of the fold UR to find the angle $\angle QRU$.
When the corner containing is folded along the line , the original angle is divided into three parts: the folded angle , the overlapping region underneath (which is also equal to due to folding symmetry), and the remaining unfolded angle shown in Figure 2.
Thus, the folding relation gives:
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