Quay lại đáp án

	2	ghi thiếu
Có: $cosC=cos(180-A-B)=-cos(A+B)=-(cosA.cosB-sinA.sinB)=sinA.sinB-cosA.cosB$ $VT=cos^2A+cos^2B+cos^2C$ $=cos^2A+cos^2B+(sinA.sinB-cosA.cosB)^2$ $=cos^2A+cos^2B+sin^2A.sin^2B+cos^2A.cos^2B-2sinA.sinB.cosA.cosB$ $=cos^2A+cos^2B+(1-cos^2A)(1-cos^2B)+cos^2A.cos^2B-2sinA.sinB.cosA.cosB$ $=cos^2A+cos^2B+1-cos^2A-cos^2B+cos^2A.cos^2B-2inA.sinB.cosA.cosB $=1-2.cosA.cosB(sinA.sinB-cosA.cosB)$ =1-2.cosA.cosB.cosC$ (đpcm) Có: $cosC=cos(180-A-B)=-cos(A+B)=-(cosA.cosB-sinA.sinB)=sinA.sinB-cosA.cosB$ $VT=cos^2A+cos^2B+cos^2C$ $=cos^2A+cos^2B+(sinA.sinB-cosA.cosB)^2$ $=cos^2A+cos^2B+sin^2A.sin^2B+cos^2A.cos^2B-2sinA.sinB.cosA.cosB$ $=cos^2A+cos^2B+(1-cos^2A)(1-cos^2B)+cos^2A.cos^2B-2sinA.sinB.cosA.cosB$ $=cos^2A+cos^2B+1-cos^2A-cos^2B+cos^2A.cos^2B-2inA.sinB.cosA.cosB$ $=1-2.cosA.cosB(sinA.sinB-cosA.cosB)$ $=1-2.cosA.cosB.cosC$ (đpcm) Có: $cosC=cos(180-A-B)=-cos(A+B)=-(cosA.cosB-sinA.sinB)=sinA.sinB-cosA.cosB$ $VT=cos^2A+cos^2B+cos^2C$ $=cos^2A+cos^2B+(sinA.sinB-cosA.cosB)^2$ $=cos^2A+cos^2B+sin^2A.sin^2B+cos^2A.cos^2B-2sinA.sinB.cosA.cosB$ $=cos^2A+cos^2B+(1-cos^2A)(1-cos^2B)+cos^2A.cos^2B-2sinA.sinB.cosA.cosB$ $=cos^2A+cos^2B+1-cos^2A-cos^2B+cos^2A.cos^2B-2inA.sinB.cosA.cosB $=1-2.cosA.cosB(sinA.sinB-cosA.cosB)$ =1-2.cosA.cosB.cosC$ (đpcm)

	1	tạo mới
Có: $cosC=cos(180-A-B)=-cos(A+B)=-(cosA.cosB-sinA.sinB)=sinA.sinB-cosA.cosB$ $VT=cos^2A+cos^2B+cos^2C$ $=cos^2A+cos^2B+(sinA.sinB-cosA.cosB)^2$ $=cos^2A+cos^2B+sin^2A.sin^2B+cos^2A.cos^2B-2sinA.sinB.cosA.cosB$ $=cos^2A+cos^2B+(1-cos^2A)(1-cos^2B)+cos^2A.cos^2B-2sinA.sinB.cosA.cosB$ $=cos^2A+cos^2B+1-cos^2A-cos^2B+cos^2A.cos^2B-2inA.sinB.cosA.cosB$ $=1-2.cosA.cosB(sinA.sinB-cosA.cosB)$ $=1-2.cosA.cosB.cosC$ (đpcm)

Chat chit và chém gió

hoahoa.nhynhay: . 11/5/2018 1:39:46 PM
hoahoa.nhynhay: . 11/5/2018 1:39:46 PM
hoahoa.nhynhay: . 11/5/2018 1:39:46 PM
hoahoa.nhynhay: . 11/5/2018 1:39:46 PM
hoahoa.nhynhay: . 11/5/2018 1:39:47 PM
hoahoa.nhynhay: . 11/5/2018 1:39:47 PM
hoahoa.nhynhay: . 11/5/2018 1:39:47 PM
hoahoa.nhynhay: . 11/5/2018 1:39:47 PM
hoahoa.nhynhay: . 11/5/2018 1:39:48 PM
hoahoa.nhynhay: . 11/5/2018 1:39:48 PM
hoahoa.nhynhay: . 11/5/2018 1:39:48 PM
hoahoa.nhynhay: . 11/5/2018 1:39:48 PM
hoahoa.nhynhay: . 11/5/2018 1:39:48 PM
hoahoa.nhynhay: . 11/5/2018 1:39:49 PM
hoahoa.nhynhay: . 11/5/2018 1:39:49 PM
hoahoa.nhynhay: . 11/5/2018 1:39:49 PM
hoahoa.nhynhay: . 11/5/2018 1:39:49 PM
hoahoa.nhynhay: . 11/5/2018 1:39:50 PM
hoahoa.nhynhay: . 11/5/2018 1:39:50 PM
hoahoa.nhynhay: ..................... 11/5/2018 1:39:52 PM
vinhlyle: hi 11/10/2018 8:03:02 PM
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quanghungnguyen256: sao wweb cứ đăng nhập mãi nhĩ, k trả lời đc bài viết nữa 11/30/2018 4:35:45 PM
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quanghungnguyen256: 11/11/2018 h là 30/11. oi web chắt k ai dùng r hả 11/30/2018 4:36:44 PM
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nguyenlena2611: 12/24/2018 9:24:22 PM
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Việt EL: ^^ 2/16/2019 8:37:21 PM
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Việt EL: y sờ e ny guan hiar? 2/16/2019 8:38:15 PM
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nln: 2/28/2019 9:02:14 PM
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dhfh: 3/2/2019 9:27:26 PM
๖ۣۜNatsu: allo 3/3/2019 11:39:32 PM
ffhfdh: reyeye 3/5/2019 8:53:26 PM
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dgdsgds: ujghjj 3/24/2019 9:12:47 PM
ryyty: ghfghgfhfhgfghgfhgffggfhhghfgh 4/9/2019 9:34:48 PM
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linhkim2401: 7/3/2019 9:35:43 AM
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cutein111: . 4/9/2020 9:23:18 PM
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minhthientran594: hi 11/1/2020 10:32:29 AM
giocon123fa: hi mọi ngừi :33 1/31/2021 10:31:56 PM
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nguyenngocha651: ai 12 tủi, sinh k9 Ib Iw mik nhố 9/21/2021 10:22:38 AM

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