79. (A) In a rectangle, the diagonals not only bisect each other but also divide the rectangle into four congruent right-angled triangles.
(R) A diagonal of a rectangle divides it into two congruent right-angled triangles, and since the diagonals are equal and bisect each other, they form four congruent triangles.
Key Concept: Diagonals of Rectangles and Squares
d) Assertion is false, but Reason is true.
[Solution Description]
In a rectangle, the diagonals are equal in length and bisect each other. This means that when the diagonals intersect, they divide each other into two equal parts.
Consider a rectangle PQRS with diagonals PR and QS intersecting at point O. Since the diagonals are equal and bisect each other, PO = OR = QO = OS. Additionally, all angles in a rectangle are $90^\circ$.
Now, let's analyze the triangles formed by the diagonals:
- Triangle POQ: It has sides PO and QO (equal in length as diagonals bisect each other), angle POQ is some angle $\theta$ (not necessarily $90^\circ$).
- Triangle QOR: It has sides QO and OR (equal in length), angle QOR is $(180^\circ - \theta)$ because POQ and QOR lie on a straight line.
However, for the triangles to be congruent, their corresponding sides and angles must be equal. Here, only the sides are equal, but the included angle between them differs unless $\theta = 90^\circ$, which would imply the diagonals are perpendicular. But in a general rectangle, the diagonals are not perpendicular (except in squares). Thus, the Assertion that the four triangles are congruent is not generally true for rectangles.
The Reason states that a diagonal divides the rectangle into two congruent right-angled triangles, which is correct. For example, triangle PQR is congruent to triangle PSR by the Side-Angle-Side (SAS) criterion (since PQ = SR, PR is common, and angle PQR = angle SRP = $90^\circ$).
But the Reason does not correctly explain why the diagonals would form four congruent triangles, as this requires specific conditions (like diagonals being perpendicular, as in squares).
Therefore, the Assertion is false (as it overgeneralizes beyond squares), but the Reason is true (as it correctly describes how a single diagonal divides the rectangle into two congruent triangles).
Your Answer is correct.
d) Assertion is false, but Reason is true.
[Solution Description]
In a rectangle, the diagonals are equal in length and bisect each other. This means that when the diagonals intersect, they divide each other into two equal parts.
Consider a rectangle PQRS with diagonals PR and QS intersecting at point O. Since the diagonals are equal and bisect each other, PO = OR = QO = OS. Additionally, all angles in a rectangle are $90^\circ$.
Now, let's analyze the triangles formed by the diagonals:
- Triangle POQ: It has sides PO and QO (equal in length as diagonals bisect each other), angle POQ is some angle $\theta$ (not necessarily $90^\circ$).
- Triangle QOR: It has sides QO and OR (equal in length), angle QOR is $(180^\circ - \theta)$ because POQ and QOR lie on a straight line.
However, for the triangles to be congruent, their corresponding sides and angles must be equal. Here, only the sides are equal, but the included angle between them differs unless $\theta = 90^\circ$, which would imply the diagonals are perpendicular. But in a general rectangle, the diagonals are not perpendicular (except in squares). Thus, the Assertion that the four triangles are congruent is not generally true for rectangles.
The Reason states that a diagonal divides the rectangle into two congruent right-angled triangles, which is correct. For example, triangle PQR is congruent to triangle PSR by the Side-Angle-Side (SAS) criterion (since PQ = SR, PR is common, and angle PQR = angle SRP = $90^\circ$).
But the Reason does not correctly explain why the diagonals would form four congruent triangles, as this requires specific conditions (like diagonals being perpendicular, as in squares).
Therefore, the Assertion is false (as it overgeneralizes beyond squares), but the Reason is true (as it correctly describes how a single diagonal divides the rectangle into two congruent triangles).