Question

triangle def is reflected across the y - axis to form triangle def. what are the coordinates of the vertices of triangle def? the coordinates of vertex d are ( ). the coordinates of vertex e are ( ). the coordinates of vertex f are ( ).

Explanation:

Step1: Recall reflection rule

When a point $(x,y)$ is reflected across the y - axis, the new point is $(-x,y)$.

Step2: Assume original coordinates

Let the original coordinates of vertices of $\triangle DEF$ be $D(x_1,y_1)$, $E(x_2,y_2)$, $F(x_3,y_3)$.

Step3: Find new coordinates

The coordinates of $D'$ will be $(-x_1,y_1)$, the coordinates of $E'$ will be $(-x_2,y_2)$ and the coordinates of $F'$ will be $(-x_3,y_3)$. But since we are not given the original coordinates, we can only state the general rule.

Answer:

If the original coordinates of $D$ is $(x_D,y_D)$, then $D'$ is $(-x_D,y_D)$; if the original coordinates of $E$ is $(x_E,y_E)$, then $E'$ is $(-x_E,y_E)$; if the original coordinates of $F$ is $(x_F,y_F)$, then $F'$ is $(-x_F,y_F)$