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    • ▾Trigonometry
      • ▸Angle measures and circles
        • •Intro to radians
        • •Arc length and sector formulas
        • •Converting between radians and degrees
        • •Degrees-minutes-seconds
      • ▸Right triangles and trig
        • •Intro to trigonometry
        • •Intro to the six basic trig functions
        • •Evaluating the six basic trig functions
        • •Evaluate the remaining basic trig functions
        • •Reciprocal and quotient identities
        • •Cofunction identities
        • •Special right triangles
        • •Area of an equilateral triangle
        • •Solving right triangles
        • •Circumradii of equilateral and isosceles triangles
      • ▸Graphing trig functions
        • •Graphing sin and cos
        • •Graphing tan and cot
        • •Graphing sec and csc
      • ▸Unit circle
        • •Finding the point on the unit circle
        • •Evaluating trig functions and solving trig equations using the unit circle
        • •Evaluating trig functions using coterminal and reference angles
        • •Proofs using the unit circle
        • •Signs of trig functions
      • ▸Inverse trig functions
        • •Composition of trig functions
        • •Evaluating inverse trig functions using the unit circle
        • •Evaluating inverse trig functions using a calculator
        • •Find the inverse of a trigonometric function
        • •Graphing inverse trigonometric functions
      • ▸Non-right triangles and trig
        • •Sine rule for area of a triangle
        • •Area of a segment
        • •Heron's formula
        • •Law of cosines
        • •Parallelogram law
        • •Stewart's theorem
        • •Law of sines
        • •Bretschneider's formula, and Brahmagupta's
      • ▾Trigonometric identities
        • •Double and half angle formulas
        • ▸Solving trig equations
          • •Solving trigonometric equations by graphing
          • •Solving trigonometric equations by factoring out the GCD
          • •Finding general solutions to trig equations
          • •Solving harder trig equations by factoring
          • •Solving trigonometric equations quadratic in form
          • •t-results
        • •Solving trig equations by factoring trinomials
        • •Sum and difference formulas
        • •Auxiliary angle method
        • •Finding the acute angle between two lines
        • •Product-to-sum and sum-to-product formulas
        • •Proving inverse trig identities
        • •Power reducing trig identities
        • •Pythagorean identities
        • •Disproving trigonometric identities by counterexample
        • •Even-odd trig identities
      • ▸Polar coordinates
        • •Plotting polar points
        • •Graphing polar equations
        • •Converting between polar and Cartesian form
      • ▸Complex numbers in polar form
        • •Multiplying and dividing complex numbers in polar form
        • •Proving de Moivre's theorem
        • •Powers of complex numbers using De Moivre's theorem
        • •Roots of complex numbers using De Moivre's theorem
      • ▸Hyperbolic and inverse hyperbolic functions
        • •Proving inverse hyperbolic identities
        • •Intro to hyperbolic functions
        • •Proving hyperbolic identities
        • •Solving hyperbolic equations
     › Trigonometry › Trigonometric identities

    Finding the acute angle between two lines

    Students will see how the difference formula for tan can be used to find the acute angle between two lines, firstly, when neither line is vertical, then when exactly one line is vertical.

    Watch these Eddie Woo videos:

    • Angle Between Two Lines (1 of 2: Calculating with radians)
    • Angle Between Two Lines (2 of 2: Dealing with vertical lines)

    Conclude by giving your students these challenges:

    • What Is the Time? by NRICH
    • Blue and White by NRICH
    • Mini-max by NRICH
    • 2007 AMC 8, Problem 12