Answer:
CD = 16
Step-by-step explanation:
Segments EF and EC are tangent and secant respectively to the circle drawn from external point E.
Thus, by tangent-secant theorem:
[tex]ED\times EC= EF^2[/tex]
[tex]\implies x(x+x+7)= (15)^2[/tex]
[tex]\implies 2x^2+7x= 225[/tex]
[tex]\implies 2x^2+7x-225=0[/tex]
Comparing above quadratic equation with [tex]ax^2+bx+c=0[/tex], we find:
a = 2, b = 7, c = -225
Now, we find the value of the discriminant
[tex]\implies b^2-4ac= (7)^2-4(2)(-225)= 1849[/tex]
Now, by quadratic formula:
[tex]x=\frac{-b\pm\sqrt{b^2-4ac}}{2a}[/tex]
[tex]\implies x=\frac{-7\pm\sqrt{1849}}{2(2)}[/tex]
[tex]\implies x=\frac{-7\pm (43)}{4}[/tex]
[tex]\implies x=\frac{-7+ 43}{4}\: or\:x=\frac{-7- 43}{4} [/tex]
[tex]\implies x=\frac{36}{4}\: or\:x=\frac{- 50}{4} [/tex]
[tex]\implies x=9\: or\:x=-12.5 [/tex]
x represents length, so it can't be negative.
[tex]\implies x\neq -12.5[/tex]
[tex]\implies x=9[/tex]
CD = x + 7
-> CD = 9 + 7
-> CD = 16
