2005
Mathematics
JAMB
If y = (1 - 2x)3 , find the value of dy/dx at x = -1
A.
57
B.
27
C.
-6
D.
-54
Correct Answer: -54
Explanation
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
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 = -54THIS WEEK's
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