SAT Subject Test Mathematics Level 1

Figure  4

As you read the stem, you might wonder what an isosceles trapezoid is. If
you’d never heard the term before, you still might have been able to
extrapolate its meaning from what you know of isosceles triangles. Isosceles
means “having two equal sides.” When applied to a trapezoid, it tells you that
the lengths of the two nonparallel sides—the legs—are equal. In this case
that’s AB and CD. If the total perimeter is 50, and the two marked sides add up
to 21 + 9 = 30, then the two unmarked sides split the difference of 50 – 30 = 20.
In other words, AB = CD = 10.

In  Figure  4,  the perimeter   of  isosceles   trapezoid   ABCD    is  50. If  BC  =   9

and AD  =   21, what    is  the length  of  diagonal    ?

1.

(A) 13

(B) 14

(C) 15

(D) 16

(E) 17