Question 13 Marks
In figure 3.81, seg EF is a diameter and seg DF is a tangent segment. The radius of the circle is r. Prove that, $\mathrm{DE} \times \mathrm{GE}=4 \mathrm{r}^2$


Answer
View full question & answer→In $\triangle \mathrm{DEF}$,
$\angle \mathrm{DFE}=90^{\circ}$ { Using tangent-radius theorem which states that a tangent at any point of a circle is perpendicular to the radius at the point of contact.}
Given: $E F=$ diameter of the circle.
$\mathrm{DE}^2=D F^2+\mathrm{EF}^2$ {Using Pythagoras theorem $\}$
$\Rightarrow D E^2=D F^2+(2 r)^2$
$\Rightarrow D E^2=D F^2+4 r^2$
$\Rightarrow D F^2=D E^2-4 r^2$
Also, $D E \times D G=D F^2$
This property is known as tangentsecant segments theorem.
$\Rightarrow D E \times D G=D E^2-4 r^2$
$\Rightarrow D E^2-D E \times D G=4 r^2$
$\Rightarrow D E(D E-D G)=4 r^2$
$\Rightarrow D E \times E G=4 r^2$
Hence, proved.
$\angle \mathrm{DFE}=90^{\circ}$ { Using tangent-radius theorem which states that a tangent at any point of a circle is perpendicular to the radius at the point of contact.}
Given: $E F=$ diameter of the circle.
$\mathrm{DE}^2=D F^2+\mathrm{EF}^2$ {Using Pythagoras theorem $\}$
$\Rightarrow D E^2=D F^2+(2 r)^2$
$\Rightarrow D E^2=D F^2+4 r^2$
$\Rightarrow D F^2=D E^2-4 r^2$
Also, $D E \times D G=D F^2$
This property is known as tangentsecant segments theorem.
$\Rightarrow D E \times D G=D E^2-4 r^2$
$\Rightarrow D E^2-D E \times D G=4 r^2$
$\Rightarrow D E(D E-D G)=4 r^2$
$\Rightarrow D E \times E G=4 r^2$
Hence, proved.

















































