MR-680 / 680. problem

Inscribed circle of trapezoid ABCD (AB || CD), the side AB touches  at point E. If AE=15, BE=10 and CD=8, calculate the area of the trapezoid.

Indicate the given distances using the identity of tangents drawn from a certain point.

Draw the two height lines of the trapezoid from points C and D.

These two height lines (blue dashed lines) divide the trapezoid into a rectangle (center) and a right-angled triangle (right-left ) share. Write your Pythagorean theorem for both right triangles.

The Pythagorean Theorem for the left triangle:

15+8-x2=h2+15-8-x2

15+8-x2-15-8-x2=h2

23-x2-7+x2=h2

529-46x+x2-49+14x+x2=h2

529-46x+x2-49-14x-x2=h2

-60x+480=h2

The Pythagorean Theorem for the right triangle:

10+x2=h2+10-x2

102+2·10·x+x2=h2+102-2·10·x+x2

102+20x+x2=h2+102-20x+x2

40x=h2

Izjedančite obe vrednosit h 2 s.

-60x+480=40x

480=40x+60x

480=100x

x=480100

x=245

h2=40·x

=40·245

=8·5·245

=192

h=192

h=64·3

=64·3

h=8·3

A=m·h

m=a+b2

=25+82

=332

A=m·h

=332·83

=332·2·43

A=1323