Logarithmic Functions

Graph and analyse logarithmic functions for IB Maths. Understand as inverses of exponentials.

y=logaxy = \log_a x is the inverse of y=axy = a^x. Understanding their graphs and properties is essential for IB Maths.

Key Features of $y = \log_a x$ ($a > 1$)

  • Domain: x>0x > 0. Range: all reals.
  • Passes through (1,0)(1, 0) and (a,1)(a, 1).
  • Vertical asymptote: x=0x = 0.
  • Increasing function.

Transformations

y=loga(xh)+ky = \log_a(x - h) + k: shift right hh, up kk. VA at x=hx = h.

Relationship with Exponentials

y=logaxy = \log_a x and y=axy = a^x are reflections in y=xy = x.

alogax=xa^{\log_a x} = x and loga(ax)=x\log_a(a^x) = x.

Worked Example

f(x)=ln(x2)+1f(x) = \ln(x - 2) + 1.

Domain: x>2x > 2. VA: x=2x = 2. Passes through (3,1)(3, 1).

Practice Problems

    1. Sketch y=log2(x+1)3y = \log_2(x+1) - 3. State domain, range, and VA.
    1. Find the inverse of f(x)=ex1+2f(x) = e^{x-1} + 2.

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Key Takeaways

  • log\log and exponential are inverses.

  • Domain of log\log: argument must be positive.

  • VA at where the argument equals zero.

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