Ta có Aa//Cb , chứng minh A B C=360* . B C A a b

PN

Phạm Nguyễn Yến Nhi

12 tháng 10 2015 - olm

Ta có Aa//Cb , chứng minh A+B+C=360* . B C A a b ...

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#Toán lớp 7

0

PG

Phạm Gia Huy

10 tháng 2 2022

Chứng minh a/a-b=c/c-d biết a/b=c/dCho ab=cd chứng minh rằng:a) aa−b =cc−db) ab=a+cb+dc)a3a+b=c3c+bd) a.cb.c=a2+c2b2+d2e) a.bc.d=a2−b2c2−d2f) a.bc.d=(a−b)2(c−d)2

#Toán lớp 7

0

HV

Hoàng Việt Đức Anh

30 tháng 6 2021

cho lăng trụ abc a'b'c' đáy là tam giác đều cạnh đáy a. A' cách đều 3 điểm A,B,C. [AA';(ABC)]=60 chứng minh B'C'CB là hình chữ nhật . tính Sxq

#Toán lớp 12

1

NV

Nguyễn Việt Lâm

Giáo viên

30 tháng 6 2021

Do A' cách đều A; B; C \(\Rightarrow\) hình chiếu vuông góc H của A' lên (ABC) trùng tâm của tam giác ABC

\(\Rightarrow\widehat{A'AH}=60^0\)

\(AH=\dfrac{2}{3}.\dfrac{a\sqrt{3}}{2}=\dfrac{a\sqrt{3}}{3}\Rightarrow AA'=\dfrac{AH}{cos60^0}=\dfrac{2a\sqrt{3}}{3}=BB'=CC'=A'B=A'C\) (do A' cách đều A, B, C nên \(A'A=A'B=A'C\))

Ta có: \(\left\{{}\begin{matrix}A'H\perp\left(ABC\right)\Rightarrow A'H\perp BC\\AH\perp BC\end{matrix}\right.\) \(\Rightarrow BC\perp\left(A'AH\right)\Rightarrow BC\perp AA'\)

\(\Rightarrow BC\perp BB'\Rightarrow B'C'CB\) là hình chữ nhật (hình bình hành có 1 góc vuông)

\(S_{BCC'B'}=BB'.BC=\dfrac{2a^2\sqrt{3}}{3}\)

Gọi M là trung điểm AB \(\Rightarrow A'M=\sqrt{A'A^2-\left(\dfrac{AB}{2}\right)^2}=\dfrac{a\sqrt[]{39}}{6}\)

\(S_{A'AB}=\dfrac{1}{2}A'M.AB=\dfrac{a^2\sqrt{39}}{12}\)

\(\Rightarrow S_{xq}=S_{BCC'B'}+4S_{A'AB}=...\)

Đúng(1)

D

dsfdsf

13 tháng 1 2020 - olm

cho tam giác abc có 3 góc nhọn. phía ngoài tam giác abc dựng các hình vuông abed, bcgf, achi có tâm lần lượt là c’, a’, b’. chứng minh: aa' = b'c' cho tam giác abc có 3 góc nhọn. phía ngoài tam giác abc dựng các hình vuông abed, bcgf, achi có tâm lần lượt là c’, a’, b’. chứng minh: aa' = b'c'cho tam giác abc có 3 góc nhọn. phía ngoài tam giác abc dựng các hình vuông abed, bcgf, achi có tâm lần lượt là c’, a’,...

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cho tam giác abc có 3 góc nhọn. phía ngoài tam giác abc dựng các hình vuông abed, bcgf, achi có tâm lần lượt là c’, a’, b’. chứng minh: aa' = b'c'

#Toán lớp 8

1

TV

Trịnh Việt Dũng

15 tháng 6 2022

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PT

Pham Trong Bach

30 tháng 4 2017

a) Cho tỉ lệ thức a b = c d . Chứng minh:i) a a + b = c c + d ; ii) a - b c - d = a + c b + d .b) Cho 2 a + b a - 2 b = 2 c + d c - 2 d . Chứng minh a b ...

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#Toán lớp 7

1

CM

Cao Minh Tâm

30 tháng 4 2017

Đúng(0)

TT

Trần Thị Thu Hường

29 tháng 9 2018 - olm

Chứng minh mọi tam giác, ta luôn có

60^o < aA+bB+cC / a+b+c < 90^o

với A, B, C là số đo (độ) các gốc tam giác và a,b,c là các cạnh đối diện với tam giác đó.

#Toán lớp 8

0

TN

Thanh Nguyễn

29 tháng 9 2019

Cho tam giác ABC.Từ điểm A,B,C dựng các vecto bằng nhau tùy ý \(\overrightarrow{AA'}=\overrightarrow{BB'}=\overrightarrow{CC'}\).Chứng...

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#Toán lớp 10

1

NV

Nguyễn Việt Lâm

Giáo viên

30 tháng 9 2019

Các kí hiệu bên dưới đều là vecto chứ ko phải đoạn thẳng:

a/ \(BB'+CC'+BA+CA=2AA'+BA+CA\)

\(=2\left(AB+BA'\right)+BA+CA=2AB+2BA'+BA+CA\)

\(=AB+CA+2BA'=CB+2BA'=CA'+A'B+2BA'\)

\(=BA'+CA'\)

b/ \(AA'+BB'+CC'=AB+BA'+BC+CB'+CA+AC'\)

\(=AB+BC+CA+BA'+CB'+AC'\)

\(=AC+CA+BA'+CB'+AC'\)

\(=BA'+CB'+AC'\)

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EC

Ex Crush

13 tháng 10 2018 - olm

Cho tam giác ABC có BC = a, CA = b, AB = c và tam giác A'B'C' có B'C' = a', C'A' = b, A'B' = c. Chứng minh rằng nếu góc A + góc A' và góc B = góc B' thì aa' = bb' + cc'.

#Toán lớp 9

0