In a rectangle, the diagonals are equal in length. Therefore, we can set the lengths of the diagonals \( AC \) and \( BD \) equal to each other:
\[ x + 29 = 4x - 1 \]
To solve for \( x \), first, let's rearrange the equation:
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Subtract \( x \) from both sides: \[ 29 = 4x - x - 1 \] Which simplifies to: \[ 29 = 3x - 1 \]
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Next, add \( 1 \) to both sides: \[ 29 + 1 = 3x \] Thus: \[ 30 = 3x \]
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Finally, divide by \( 3 \): \[ x = 10 \]
So, the value of \( x \) that makes \( ABCD \) a rectangle is \( x = 10 \).