If y 1 - 2x 2 find the value of dy dx at x -1 - JAMB Mathematics 2005 Question

y = (1 - 2x)³

let u = 1-2x

du/dx = -2

∴y = u³

dy/du = 3u²

But dy/dx = du/dx * dy/du

= -2 * 3u²

= -6u²

= -6(1-2x)²

At x = -1: dy/dx = -6(1 - 2(-1))²

dy/dx = -6(1 + 2)²

= -6 * 3²

= -6 * 9

= -54