Ticks into Equations

Part of the Triangles section of Coddy's Geometry journey. Lesson 4 of 71.

Tick marks tell you that two sides have the same length. When the ticked sides are labelled with expressions instead of numbers, you can write that fact as an equation. In the figure, the ticked sides are labelled 3x - 2 and 10.

$An isosceles triangle whose ticked sides are labelled 3x minus 2 and 10, with base 7$

Matching ticks mean equal lengths

The ticks say that the two labels are equal, so write the equation:

3x - 2 = 10

Add 2 to both sides to get 3x = 12, then divide both sides by 3 to get x = 4. The base, 7, has no tick, so it does not appear in the equation. The equation only uses the sides that the ticks mark as equal.

Before you write anything, look at the figure. Find the sides that carry the same marks, and read the label on each of them. The equation always comes from the marks. Two sides that look equal in the drawing are not equal unless they are marked. Once you have written the equation, solve it the same way as any other equation.