Solve for : .
Solve for $x$:
$\begin{vmatrix} 1 & -2 & 5 \\ 2 & x & -1 \\ 0 & 4 & 2x \end{vmatrix} = 86$.
Answer
Answer
AIWritten by AI (gemini-2.5-pro) - it can contain mistakes.
Expanding the determinant along the first column:
$1(2x^2 - (-4)) - 2(-4x - 20) + 0 = 86$
$2x^2 + 4 + 8x + 40 = 86$
$2x^2 + 8x - 42 = 0$
$x^2 + 4x - 21 = 0$
$(x + 7)(x - 3) = 0$
$x = 3$ or $x = -7$
From ISC 2026 Mathematics Paper 1, question 1(xiii).