I used to despise constructions back in school

Geometry Level 3

We want to construct a trianlge A B C ABC satisfying the given conditions: A = 3 0 , A B = 95 , B C = 55 A=30^\circ, AB=95, BC=55 .

How many such triangles A B C ABC exist up to congruence?


The answer is 2.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Otto Bretscher
May 10, 2015

Let's go back to school and treat this as a construction problem ;)

We can draw the line segment AB = 95, and we draw a ray R emanating from A, making an angle of 3 0 o 30^o with AB, there C will be placed. The distance between R and B is sin ( 3 0 o ) × 95 = 47.5 \sin(30^o)\times95=47.5 , so that the circle about B with radius 55 will intersect R twice (since 55 < 95 55<95 ), giving us our 2 \boxed{2} solutions C.

Another possible approach is to use Law of Cosines:

B C 2 = A B 2 + A C 2 2 A B A C cos ( A ) 5 5 2 = 9 5 2 + A C 2 2 ( 95 ) ( A C ) cos ( 3 0 ) A C 2 95 3 A C + 6000 = 0 BC^2=AB^2+AC^2-2AB\cdot AC \cos(\angle A) \\ 55^2=95^2+AC^2-2(95)(AC)\cos(30^\circ) \\ AC^2-95\sqrt{3}AC+6000=0

If we compute the discriminant of that quadratic we get:

Δ = ( 95 3 ) 2 4 ( 1 ) ( 6000 ) = 3075 \Delta=(-95\sqrt{3})^2-4(1)(6000)=3075

We got that Δ > 0 \Delta>0 , so there are 2 \boxed{2} distinct real solutions for A C AC , and by Descartes Rule of Signs, both are positive.

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...