JAMB - Mathematics (2005 - No. 28)
If y = (1 - 2x)\(^3\), find the value of dy/dx at x = -1
57
27
-6
-54
Explanation
y = (1 - 2x)\(^3\)
let u = 1-2x
du/dx = -2
∴y = u\(^3\)
dy/du = 3u\(^2\)
But dy/dx = du/dx * dy/du
= -2 * 3u\(^2\)
= -6u\(^2\)
= -6(1-2x)\(^2\)
At x = -1: dy/dx = -6(1 - 2(-1))\(^2\)
dy/dx = -6(1 + 2)\(^2\)
= -6 * 3\(^2\)
= -6 * 9
= -54
let u = 1-2x
du/dx = -2
∴y = u\(^3\)
dy/du = 3u\(^2\)
But dy/dx = du/dx * dy/du
= -2 * 3u\(^2\)
= -6u\(^2\)
= -6(1-2x)\(^2\)
At x = -1: dy/dx = -6(1 - 2(-1))\(^2\)
dy/dx = -6(1 + 2)\(^2\)
= -6 * 3\(^2\)
= -6 * 9
= -54
Comments (0)
