Example 1: Chain rule with a logarithm
Find dy/dx if y = ln(3x + 1).
- Let
u = 3x + 1, soy = ln u dy/du = 1/u,du/dx = 3dy/dx = (1/u) × 3
Answer: dy/dx = 3 / (3x + 1)
M1 differentiation uses four rules (addition, product, quotient, chain) and six formulas: (xⁿ)′ = nxⁿ⁻¹, (eˣ)′ = eˣ, (ln x)′ = 1/x, (aˣ)′ = aˣ ln a, (loga x)′ = 1/(x ln a) and (C)′ = 0. Trigonometric functions are not part of M1.
(xⁿ)′ = nxⁿ⁻¹(eˣ)′ = eˣ, (aˣ)′ = aˣ ln a(ln x)′ = 1/x, (loga x)′ = 1/(x ln a)(uv)′ = u′v + uv′(u/v)′ = (u′v − uv′) / v²dy/dx = (dy/du)(du/dx)Find dy/dx if y = ln(3x + 1).
u = 3x + 1, so y = ln udy/du = 1/u, du/dx = 3dy/dx = (1/u) × 3Answer: dy/dx = 3 / (3x + 1)
Differentiate y = (x² + 1)e³ˣ.
u = x² + 1, u′ = 2x; v = e³ˣ, v′ = 3e³ˣdy/dx = 2x e³ˣ + (x² + 1)(3e³ˣ)= e³ˣ(3x² + 2x + 3)Answer: dy/dx = e³ˣ(3x² + 2x + 3)
(e³ˣ)′ = 3e³ˣ and (ln(3x + 1))′ = 3/(3x + 1). Leaving out the factor from the inside function is the most frequent M1 differentiation error.(2ˣ)′ = 2ˣ ln 2, not x · 2ˣ⁻¹. The power rule applies only when the variable is the base.How to study: Click any question below to instantly load it on the coordinate grid and check your steps.