a: Xét ΔAMB và ΔAMC có 

AM chung

MB=MC

AB=AC

Do đó: ΔAMB=ΔAMC

Bài 7:

h: =-2005-21+75+2005=54

Bài 4: 

a: \(A=2x^4+3x^3-5x^2+2x+5\)

\(B=x^4+3x^3-6x^2+2x+1\)

b: \(A+B=3x^4+6x^3-11x^2+4x+6\)

\(A-B=x^4+x^2+4\)

c: \(A-B=x^4+x^2+4>=4>0\forall x\)

\(A+B=x^3-3x^2y+2xy^2-5y^3-x^2y+2xy^2+4x^3-x^2y+2xy^2+y^3-2x^3-4xy^2.=3x^3-4y^3-5x^2y+2xy^2.\)

\(A-B=x^3-3x^2y+2xy^2-5y^3-x^2y+2xy^2-4x^3+x^2y-2xy^2-y^3+2x^3+4xy^2=-x^3-6y^3-3x^2y+6xy^2.\)

went

are practicing

does-do-goes

to go

ask

watching

doing

to do

will become

was

1.went

2.are practicing

3.does-do-goes

4.to go

5.ask

6.watching

7.doing

8.to do

9.will become

10.was

Câu 1: D

Câu 2: D

D

C

D

Nhìn như ktr

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