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In triangles ABC and DEF, ∠C = ∠F, AC = DF, and BC = EF. If AB = 2x - 1 and DE = 5x - 4, then the value of x is

  • A.1
  • B.2
  • C.3
  • D.4

Answer: A

The value of x is 1.

In the two triangles, AC = DF, BC = EF, and the included angles ∠C and ∠F are equal. By the SAS (side-angle-side) congruence rule, triangle ABC is congruent to triangle DEF, so the third sides are equal: AB = DE.

\[ \begin{aligned} 2x - 1 &= 5x - 4 \\ 3 &= 3x, \quad x = 1 \end{aligned} \]

Check: AB = 2(1) - 1 = 1 and DE = 5(1) - 4 = 1. Any other value of x would make AB and DE unequal, which the congruence rules out.

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