Cho ▲ABC có <A=90o đường cao AH, BH=4cm,CH=9cm
Tính a) AB,AH
b) <B,AC
c)SABC
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
Bài 3: Đặt \(\hat{A}=a;\hat{B}=b;\hat{C}=c\)
Xét ΔABC có \(\hat{A}+\hat{B}+\hat{C}=180^0\)
=>a+b+c=180
Ta có: \(\hat{C}-3\cdot\hat{B}-2\cdot\hat{A}=-3^0\)
=>c-3b-2a=-3
=>2a+3b-c=3
mà a+b+c=180
nên 2a+3b-c+a+b+c=3+180
=>3a+4b=183
=>6a+8b=366
\(5\cdot\hat{B}-2\cdot\hat{A}=16^0\)
=>5b-2a=16
=>15b-6a=48
=>15b-6a+6a+8b=366+48
=>23b=414
=>\(b=\frac{414}{23}=18^0\)
=>\(\hat{B}=18^0\)
3a+4b=183
=>3a=183-4b=183-72=111
=>\(a=\frac{111}{3}=37^0\)
=>\(\hat{A}=37^0\)
\(\hat{C}=180^0-18^0-37^0=180^0-55^0=125^0\)
Bài 2:
Đặt \(\hat{A}=a;\hat{B}=b;\hat{C}=c\)
Xét ΔABC có \(\hat{A}+\hat{B}+\hat{C}=180^0\)
=>a+b+c=180
\(\hat{A}+\hat{B}-2\cdot\hat{C}=27^0\)
=>a+b-2c=27
=>(a+b+c)-(a+b-2c)=180-27
=>3c=153
=>\(c=\frac{153}{3}=51\)
=>\(\hat{C}=51^0\)
\(\hat{A}+3\cdot\hat{C}=273^0\)
=>\(\hat{A}=273^0-3\cdot51^0=273^0-153^0=120^0\)
\(\hat{B}=180^0-51^0-120^0=60^0-51^0=9^0\)
bài 1:
Đặt \(\hat{A}=a;\hat{B}=b;\hat{C}=c\)
Xét ΔABC có \(\hat{A}+\hat{B}+\hat{C}=180^0\)
=>a+b+c=180
\(\hat{A}-\hat{B}+\hat{C}=90^0\)
=>a-b+c=90
=>a+b+c-(a-b+c)=180-90
=>2b=90
=>b=45
=>\(\hat{B}=45^0\)
=>\(\hat{A}+\hat{C}=180^0-45^0=135^0\)
mà \(\hat{A}-\hat{C}=-5^0\)
nên \(\hat{A}=\frac{135^0-5^0}{2}=\frac{130^0}{2}=65^0\)
=>\(\hat{C}=135^0-65^0=70^0\)
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: (SB;(ABC))=(SB;BA)=góc SBA
\(\tan SBA=\dfrac{SA}{AB}=\sqrt{6}\)
=>góc SBA=68 độ
b: (SA;(SBC))=(SA;SB)=góc ASB
tan ASB=AB/SA=1/căn 6
=>góc ASB=22 độ
hình:
A B C H
~~~
a/ Ta có: BC = BH + CH = 4 + 9 = 13(cm)
a/d hệ thức lượng trong tam giác ABC vuông tại A có:
\(\left\{{}\begin{matrix}AB^2=BC\cdot BH=13\cdot4=52\\AH^2=BH\cdot CH=9\cdot4=36\end{matrix}\right.\)
\(\Rightarrow\left\{{}\begin{matrix}AB\approx7,2\left(cm\right)\\AH=6\left(cm\right)\end{matrix}\right.\)
b/ Ta có: cosB = \(\dfrac{AB}{BC}=\dfrac{7,2}{13}\Rightarrow\widehat{B}=34^o\)
a/d pitago vào tam giác ABC có:
\(BC^2=AB^2+AC^2\Rightarrow AC=\sqrt{BC^2-AB^2}=\sqrt{13^2-7,2^2}\approx10,8\left(cm\right)\)
c/ \(S_{ABC}=\dfrac{1}{2}AH\cdot BC=\dfrac{1}{2}\cdot6\cdot13=39\left(cm^2\right)\)