Cho tam giác ABC có A( 1; -2) ,đường cao CH: x-y +1= 0,đường phân giác trong BN: 2x + y+ 5 = 0.Tọa độ điểm B là:
A.(2 ; 3)
B.(4 ; -5)
C.(-6 ; 4)
D.(- 4 ; 3)
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Bài 1:
a: Xét ΔABC có \(AC^2=AB^2+BC^2\)
nên ΔABC vuông tại B
b: XétΔABC có BC<AB<AC
nên \(\widehat{A}< \widehat{C}< \widehat{B}\)
Tọa độ điểm C:
\(\left\{{}\begin{matrix}x_C=3x_I-x_A-x_B=1\\y_C=3y_I-y_A-y_B=-4\end{matrix}\right.\Rightarrow C\left(1;-4\right)\)
Ta có:
\(\overrightarrow{AH}=\left(a-3;b+1\right)\)
\(\overrightarrow{BH}=\left(a+1;b-2\right)\)
\(\overrightarrow{BC}=\left(2;-6\right)\)
\(\overrightarrow{AC}=\left(-2;-3\right)\)
Theo giả thiết
\(AH\perp BC\Rightarrow2\left(a-3\right)-6\left(b+1\right)=0\Leftrightarrow a-3b=6\left(1\right)\)
\(BH\perp AC\Rightarrow-2\left(a+1\right)-3\left(b-2\right)=0\Leftrightarrow2a+3b=4\left(2\right)\)
Từ \(\left(1\right);\left(2\right)\Rightarrow\left\{{}\begin{matrix}a=\dfrac{10}{3}\\b=-\dfrac{8}{9}\end{matrix}\right.\Rightarrow a+3b=\dfrac{2}{3}\)
Gọi tọa độ điểm H(a;b)
Ta có: A H → = a + 1 ; b − 1 , B H → = a ; b − 2 , B C → = 1 ; − 1 , A C → 2 ; 0
Do H là trực tâm tam giác ABC nên:
A C → . B H → = 0 B C → . A H → = 0 ⇒ 2. a + 0. b − 2 = 0 1. a + 1 − 1. b − 1 = 0 ⇒ a = 0 b = 2
Vậy H (0; 2).
Chọn A
\(AB=\sqrt{\left(-2-2\right)^2+\left(-1+2\right)^2}=\sqrt{17}\)
\(AC=\sqrt{\left(1-2\right)^2+\left(2+2\right)^2}=\sqrt{17}\)
Vậy tam giác ABC cân tại A.
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Đáp án D
Ta có AB và CH vuông góc với nhau nên AB qua A(1; -2) và nhận VTPT ( 1;1). Phương trình AB:
1(x-1) + 1( y+2) = 0 hay x+ y +1 = 0
Có AB và BN cắt nhau tại B nên tọa độ B là nghiệm hệ phương trình