cho tam giác ABC, góc B= góc C, A<90 độ, từ A kẻ AH vuông góc BC ( H thuộc BC ). Trên cạnh AB lấy điểm L, trên cạnh AC lấy điểm K sao cho AL=AK.
a,C/M góc BAH= góc CAH
b, C/M góc BKC= góc CLB
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3)- theo bài tao có :A+B+C=180 độ.(định lí tổng ba góc của 1 tam giác)
C:B:A=1:3:6 => C/1=B/3=A/6=(A+B+C)/(1+3+6)=180/10=18
Do đó :C/1=18 B/3=18 A/6=18
=>C=18 độ =>B=54 độ =>A=104 độ
a: Ta có: \(\widehat{C}=\dfrac{\widehat{B}}{2}=\dfrac{\widehat{A}}{3}\)
\(\Leftrightarrow\left\{{}\begin{matrix}\widehat{B}=2\cdot\widehat{C}\\\widehat{A}=3\cdot\widehat{C}\end{matrix}\right.\)
Xét ΔABC có
\(\widehat{A}+\widehat{B}+\widehat{C}=180^0\)
\(\Leftrightarrow6\cdot\widehat{C}=180^0\)
\(\Leftrightarrow\widehat{C}=30^0\)
Suy ra: \(\widehat{A}=90^0\)
Xét ΔABC có \(\widehat{A}=90^0\)
nên ΔABC vuông tại A
b: Ta có: \(\widehat{B}+\widehat{C}=90^0\)
\(\widehat{HAC}+\widehat{C}=90^0\)
Do đó: \(\widehat{B}=\widehat{HAC}\)
Ta có: \(\widehat{B}+\widehat{C}=90^0\)
\(\widehat{BAH}+\widehat{B}=90^0\)
Do đó: \(\widehat{C}=\widehat{BAH}\)
3:
góc C=90-50=40 độ
Xét ΔABC vuông tại A có sin C=AB/BC
=>4/BC=sin40
=>\(BC\simeq6,22\left(cm\right)\)
\(AC=\sqrt{BC^2-AB^2}\simeq4,76\left(cm\right)\)
1:
góc C=90-60=30 độ
Xét ΔABC vuông tại A có
sin B=AC/BC
=>3/BC=sin60
=>\(BC=\dfrac{3}{sin60}=2\sqrt{3}\left(cm\right)\)
=>\(AB=\dfrac{2\sqrt{3}}{2}=\sqrt{3}\left(cm\right)\)
a: Áp dụng tính chất của dãy tỉ số bằng nhau, ta được:
\(\dfrac{a}{1}=\dfrac{b}{3}=\dfrac{c}{5}=\dfrac{a+b+c}{1+3+5}=\dfrac{180}{9}=20\)
Do đó: a=20; b=60; c=100
Vậy: ΔABC là tam giác tù
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Lời giải:
a)
$\widehat{B}=\widehat{C}(1)$
$\widehat{AHB}=\widehat{AHC}=90^0(2)$ (do $AH\perp BC$)
$\widehat{B}+\widehat{AHB}+\widehat{BAH}=\widehat{C}+\widehat{AHC}+\widehat{CAH}=180^0(3)$ (tính chất tổng 3 góc trong 1 tam giác)
Từ $(1);(2);(3)\Rightarrow \widehat{BAH}=\widehat{CAH}$ (đpcm)
b)
Vì $\widehat{B}=\widehat{C}$ nên tam giác $ABC$ cân tại $A$
$\Rightarrow $AB=AC$. Mà $AL=AK$ nên $AB-AL=AC-AK$ hay $BL=CK$
Xét tam giác $BKC$ và $CLB$ có:
$BC$ chung
$KC=LB$ (cmt)
$\widehat{B}=\widehat{C}$ (gt)
$\Rightarrow \triangle BKC=\triangle CLB$ (c.g.c)
$\Rightarrow \widehat{BKC}=\widehat{CLB}$
a) Xét ΔABC có \(\widehat{ABC}=\widehat{ACB}\)(gt)
nên ΔABC cân tại A(Định lí đảo của tam giác cân)
hay AB=AC
Xét ΔABH vuông tại H và ΔACH vuông tại H có
AB=AC(cmt)
AH chung
Do đó: ΔABH=ΔACH(cạnh huyền-cạnh góc vuông)
Suy ra: \(\widehat{BAH}=\widehat{CAH}\)(hai góc tương ứng)