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Equation or Identity (2)

Age 16 to 18 Challenge Level:

This resource is from Underground Mathematics.

Throughout, $A$, $B$ and $C$ are the angles of a triangle.

For each of the following, decide whether it is an identity (true for all triangles) or an equation (there is a triangle for which it is not true).

If it is an identity, true for all triangles, then you should prove it (using trigonometric identities that you already know).

If it is an equation, then give an example of a triangle for which it is not true.  You could also try to solve the equation (that is, find all triangles for which it is true).

1.  $\sin(A + 2B) = \sin A + 2\sin B \cos(A + B)$.

2.  $\tan(A - B) + \tan(B - C) + \tan(C - A) = 0$.

3.  $2\sin A \cos^2\left(\frac{B}{2}\right) + 2\cos^2\left(\frac{A}{2}\right)\sin B = \sin(A + B) + \sin(B + C) + \sin(C + A)$.

4.  $\sin(A+B) = \cos C$.

5.  $\cos C = -\cos(A+B)$.

6.  $4(\cos^2 A \cos^2 B + \sin^2 A \sin^2 B) - 2 \sin(2A) \sin(2B) = 3$.

7.  $\sin(2A) + \sin(2B) + \sin(2C) = 4\sin A \sin B \sin C$.

This is an Underground Mathematics resource.

Underground Mathematics is funded by a grant from the UK Department for Education and provides free web-based resources that support the teaching and learning of post-16 mathematics. It started in 2012 as the Cambridge Mathematics Education Project (CMEP).

Visit the site at to find more resources, which also offer suggestions, solutions and teacher notes to help with their use in the classroom.