Given △ A B C . I is the incenter of △ A B C , D ; E are the mipoints of A C ; A B ,respectively. D I intersects A B at Q and E I intersects A C at P .
We know that S A B C = S A P Q , find ∠ B A C in degree.
Clarification : S A B C and S A P Q denote the area, respectively, of the triangles A B C and A P Q .
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.
First of all, we note the following facts: 1 . 2 ⋅ S A B C = A B ⋅ A C sin ∠ B A C 2 . 2 ⋅ S A P Q = A P ⋅ A Q sin ∠ B A C 3 . 0 ≤ A P ≤ A C 4 . 0 ≤ A Q ≤ A B
Note that 3 . and 4 . imply that A P ⋅ A Q ≤ A B ⋅ A C ( 5 . ) with equality holding only if it holds in both 3 . and 4 . .
The question states that S A B C = S A P Q . Hence, equality holds in 5 . . Hence, equality also holds in 3 . and 4 . , that is, P = C and Q = B . Therefore, the incentre and the centroid of Δ A B C coincide.
This implies that Δ A B C is equilateral. Hence ∠ B A C is 6 0 ∘ .
Problem Loading...
Note Loading...
Set Loading...
The areas can only be equal if B and Q coincides since A, B, Q are collinear. Similarly C coincides with P.
That means Median PE and QD, that is CE and BD meet at centroid I.
Centroid and incenter coincides.
That is possible only in equilateral triangle.
Angle BAC=60.