Tam giác ABC có góc A = 400 ; các đường cao cắt nhau tại H.Khi đó số đo góc BHC bằng ......
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Xét tam giác ABC có:
\(\widehat{A}+\widehat{B}+\widehat{C}=180^0\)( tổng 3 góc trong tam giác)
\(\Rightarrow\widehat{B}+\widehat{C}=180^0-\widehat{A}=180^0-70^0=110^0\)
Xét tam giác ABC có:
\(\left\{{}\begin{matrix}\widehat{B}+\widehat{C}=110^0\\\widehat{B}-\widehat{C}=40^0\end{matrix}\right.\)
\(\Rightarrow\left\{{}\begin{matrix}\widehat{B}=\left(110^0+40^0\right):2=75^0\\\widehat{C}=\left(110^0-40^0\right):2=35^0\end{matrix}\right.\)
Ta có: \(\widehat{C}< \widehat{A}< \widehat{B}< 90^0\)
Vậy tam giác ABC là tam giác nhọn
tam giác ABM và tam giác ACM có: AB = AC (GT) góc ABM = góc ACM (vì AB = AC => tam giác ABC cân) BM = MC (GT) => tam giác ABM = tam giác ACM (c.g.c) => ˆ A M B = ˆ A M C (2 góc tương ứng) Mà ˆ A M B + ˆ A M C =1800 (kề bù) => ˆ A M B = ˆ A M C = 1 2 1800 = 900 Vậy ˆ A M B =900 ; ˆ A M C =900
1.
\(A+B+C=180^0\Rightarrow A=180^0-\left(B+C\right)=70^0\)
Kẻ đường cao BD
Trong tam giác vuông ABD:
\(cotA=\dfrac{AD}{BD}\Rightarrow AD=BD.cotA\)
Trong tam giác vuông BCD:
\(cotC=\dfrac{CD}{BD}\Rightarrow CD=BD.cotC\)
\(\Rightarrow AD+CD=BD.cotA+BD.cotC\)
\(\Rightarrow AC=BD.\left(cotA+cotC\right)\)
\(\Rightarrow BD=\dfrac{AC}{cotA+cotC}\)
\(\Rightarrow S_{ABC}=\dfrac{1}{2}BD.AC=\dfrac{1}{2}.\dfrac{AC^2}{cotA+cotC}=\dfrac{35^2}{2\left(cot70^0+cot50^0\right)}\approx509,1\left(cm^2\right)\)
tu ve hinh
tamgiac ABC can tai A (gt)
=> goc ABC = goc ACB = (1800 - goc A) : 2 (tc) ma goc A = 40o (gt)
=> goc ABC = goc ACB = 700
Câu hỏi của Nguyễn Anh Thư - Toán lớp 7 - Học toán với OnlineMath
Bạn vào ô công thức để nhập lại số đo góc đi bạn. Khó hiểu quá
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∠ B = ∠ C = ( 180 0 – 40 0 ) / 2 = 70 0
Mà ∠ M 2 + ∠ B = 180 0 (hai góc trong cùng phía nên bù nhau)
Suy ra: ∠ M 2 = 180 0 - ∠ B = 180 0 – 70 0 = 110 0
∠ N 2 = ∠ M 2 = 110 0 (tính chất hình thang cân)