ABC ~ DEF
What is the value for x, the length of side BC?

17.5

Step-by-step explanation:

as the triangles are similar, when oriented in the same direction they have the same angles, and the lengths of all sides of DEF are the lengths of the sides of ABC but multiplied by the same scaling factor f for all sides.

so, we see that

A ~ D

B ~ E

C ~ F

and therefore

AB ~ DE

BC ~ EF

CA ~ FD

that means

DE = AB × f

EF = BC × f

FD = CA × f

we know DE and AB.

so,

4 = 10 × f

f = 4/10 = 2/5

and now we know

EF = 7 = BC × f

BC = 7/f = (7/1) / (2/5) = (5×7)/(2×1) = 35/2 = 17.5

in geometry, parallel lines are lines in a plane which do not meet; that is, two lines in a plane that do not intersect or touch each other at any point are said to be parallel. by extension, a line and a plane, or two planes, in three-dimensional euclidean space that do not share a point are said to be parallel.