Question:medium

In triangles ABC and PQR, \(\angle A = \angle Q\) and \(\angle B = \angle R\), then AB : AC is equal to :

Show Hint

To find side ratios without confusion, write the similarity statement \(\Delta ABC \sim \Delta QRP\) clearly first.
Then, pick the pairs of letters directly:
- \(AB\) (letters 1 and 2) corresponds to \(QR\) (letters 1 and 2).
- \(AC\) (letters 1 and 3) corresponds to \(QP\) (letters 1 and 3).
This gives \(\frac{AB}{AC} = \frac{QR}{QP}\) immediately!
Updated On: Jul 7, 2026
  • PQ : PR
  • PQ : QR
  • QR : QP
  • PR : QR
Show Solution

The Correct Option is C

Solution and Explanation

Step 1: Match up the vertices from the equal angles.
We are told $\angle A = \angle Q$ and $\angle B = \angle R$. Since the three angles of a triangle always add to $180^\circ$, the remaining angles must also match: $\angle C = \angle P$. So the correspondence is $A \leftrightarrow Q$, $B \leftrightarrow R$, $C \leftrightarrow P$, giving $\triangle ABC \sim \triangle QRP$.

Step 2: Use the Sine Rule inside triangle $ABC$ instead of the similarity-ratio rule.
In any triangle, a side is proportional to the sine of the angle opposite it. In $\triangle ABC$:
\[ \frac{AB}{\sin C} = \frac{AC}{\sin B} \]
Rearranging:
\[ \frac{AB}{AC} = \frac{\sin C}{\sin B} \]

Step 3: Use the Sine Rule inside triangle $QRP$.
Since $\angle Q$ is opposite side $RP$, and $\angle R$ is opposite side $QP$, in $\triangle QRP$:
\[ \frac{QR}{\sin P} = \frac{QP}{\sin R} \]
Rearranging:
\[ \frac{QR}{QP} = \frac{\sin P}{\sin R} \]

Step 4: Substitute the equal angles.
We know $\angle P = \angle C$ (from Step 1) and $\angle R = \angle B$ (given). Substituting these into the ratio from Step 3:
\[ \frac{QR}{QP} = \frac{\sin C}{\sin B} \]

Step 5: Compare with the ratio from triangle $ABC$.
From Step 2, $\frac{AB}{AC} = \frac{\sin C}{\sin B}$, which is the exact same expression found in Step 4 for $\frac{QR}{QP}$. So the two ratios must be equal.

Step 6: Final Answer.
$AB : AC$ is equal to $QR : QP$, so option (C) is correct. \[ \boxed{AB : AC = QR : QP} \]
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