What is the length of dc? in this diagram, δabc ~ δedc.
a. 3
b. 4
c. 6
d. 5

D) 5

Explanation:
Since the two triangles are similar:
therefore:
AC/CE = BC/CD
We are given that:
AC = 21
CE = 7
BC = 20-x
CD = x
Substitute with the givens in the above relation and solve for x as follows:
AC/CE = BC/CD
21/7 = (20-x) / x
3 = (20-x)/x
3x = 20-x
3x+x = 20
4x = 20
x = 20/4
x = 5
Now we have:
BC = 20-x = 20-5 = 15 
CD = x = 5

Hope this helps :)

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