如图所示,一张平行四边形纸片 PQRS 沿着两条虚线 TS 和 UR 折叠,如图 1 所示。在图 2 中,∠PST=32∘\angle PST = 32^\circ 且 ∠QRS=14∘\angle QRS = 14^\circ。

  1. (a)

    求图 2 中的 ∠UQR\angle UQR。

  2. (b)

    求图 2 中的 ∠QRU\angle QRU。

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完整解法

10(a)

1. Relate the folded angle $\angle UQR$ to the original angle of the parallelogram.

When the paper is folded, the angle ∠UQR\angle UQR in Figure 2 corresponds directly to the original interior angle ∠PQR\angle PQR 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:
∠PQR=∠PSR\angle PQR = \angle PSR

2. Calculate the original interior angle $\angle PSR$ to find $\angle UQR$.

The fold line TSTS is perpendicular to the parallel horizontal sides PQPQ and SRSR of the parallelogram. Therefore, the angle between the vertical line TSTS and the base SRSR is 90∘90^\circ (i.e., ∠TSR=90∘\angle TSR = 90^\circ).

The full angle at SS before folding, ∠PSR\angle PSR, is the sum of the angle of the folded part and the right angle:
∠PSR=∠PST+∠TSR=32∘+90∘=122∘\angle PSR = \angle PST + \angle TSR = 32^\circ + 90^\circ = 122^\circ

Thus,
∠UQR=∠PSR=122∘\angle UQR = \angle PSR = 122^\circ

答案

122∘122^\circ

10(b)

1. Find the interior angles of the original parallelogram PQRS.

In the original parallelogram PQRSPQRS, we are given in the folding diagram that after folding along the vertical line TSTS, the remaining angle is 32∘32^\circ.
Since TS⊥PQTS \perp PQ, in the unfolded state, the angle at SS of the right-angled triangle is 90∘−58∘=32∘90^\circ - 58^\circ = 32^\circ.
Therefore, the original angle ∠PSR\angle PSR of the parallelogram is:
∠PSR=90∘+32∘=122∘\angle PSR = 90^\circ + 32^\circ = 122^\circ
Since adjacent angles in a parallelogram sum to 180∘180^\circ, the original angle ∠QRS\angle QRS is:
∠QRSoriginal=180∘−122∘=58∘\angle QRS_{\text{original}} = 180^\circ - 122^\circ = 58^\circ

2. Use the symmetry of the fold UR to find the angle $\angle QRU$.

When the corner containing QQ is folded along the line URUR, the original angle ∠QRSoriginal\angle QRS_{\text{original}} is divided into three parts: the folded angle ∠QRU\angle QRU, the overlapping region underneath (which is also equal to ∠QRU\angle QRU due to folding symmetry), and the remaining unfolded angle ∠QRSremaining=14∘\angle QRS_{\text{remaining}} = 14^\circ shown in Figure 2.

Thus, the folding relation gives:
2×∠QRU+14∘=∠QRSoriginal2 \times \angle QRU + 14^\circ = \angle QRS_{\text{original}}
2×∠QRU+14∘=58∘2 \times \angle QRU + 14^\circ = 58^\circ
2×∠QRU=58∘−14∘=44∘2 \times \angle QRU = 58^\circ - 14^\circ = 44^\circ
∠QRU=22∘\angle QRU = 22^\circ

答案

52∘52^\circ

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