\(\frac{3a+7b}{7a+3b}=1\)

=> 3a + 7b = 7a + 3b

=> 3a - 3b = 7a - 7b 

=> 3(a - b) = 7(a - b)

=> 7(a - b) - 3(a - b) = 0

=> 4(a - b) = 0

=> a - b = 0

=> a = b

Ta có : A = -a + b - 1

=> A = -a + a - 1

=> A = -1

cick mình nha bạn

3a+7b=7a+3b => a=b

-a+b-1=-a+a-1=-1

Câu 5:

\(D\left(2\right)=21a+9b-6a-4b\)

\(D\left(2\right)=\left(21a-6a\right)+\left(9b-4b\right)\)

\(D\left(2\right)=15a+5b\)

Mà: \(3a+b=18\Rightarrow b=18-3b\)

\(\Rightarrow D\left(2\right)=15a+5\left(18-3b\right)\)

\(D\left(2\right)=15a+90-15a\)

\(D\left(2\right)=90\)

Vậy: ...

Câu 4:

\(D\left(1\right)=4a+10b-b+2a\)

\(D\left(1\right)=\left(4a+2a\right)+\left(10b-b\right)\)

\(D\left(1\right)=6a+9b\)

Mà: \(2a+3b=12\Rightarrow a=\dfrac{12-3b}{2}\)

\(\Rightarrow D\left(1\right)=6\left(\dfrac{12-3b}{2}\right)+9b\)

\(D\left(1\right)=\dfrac{6\left(12-3b\right)}{2}+9b\)

\(D\left(1\right)=3\left(12-3b\right)+9b\)

\(D\left(1\right)=36-9b+9b\)

\(D\left(1\right)=36\)

Vậy: ...

Câu 3:

Sửa đề: \(C=5a-4b+7a-8b\)

\(C=\left(5a+7a\right)-\left(4b+8b\right)\)

\(C=12a-12b\)

\(C=12\left(a-b\right)\)

\(C=12\cdot8\)

\(C=96\)

Vậy: ...

4:

D=6a+9b=3(2a+3b)=36

5: 

D=15a+5b=5(3a+b)=90

Câu 5:
D=21a+9b-6a-4b

=21a-6a+9b-4b

=15a+5b

=5(3a+b)

\(=5\cdot18=90\)

Câu 4: D=4a+10b-b+2a

=4a+2a+10b-b

=6a+9b

=3(2a+3b)

\(=3\cdot12=36\)

Câu 3:

C=5a-4b+7a+8

=5a+7a-4b+8

=12a-12b+8b+8

=12(a-b)+8b+8

=8(a-b)+8b+8

=8a-8b+8b+8

=8a+8

1: C=4a+2a+10b-b

=6a+9b

=3(2a+3b)

=3*12=36

D=21a+9b-6a-4b

=15a+5b

=5(3a+b)

=5*18=90

B=5a+7a-4b-8b

=12a-12b

=12(a-b)

=12*8=96

4:

Gọi hai số cần tìm là a,b

Theo đề, ta có hệ phương trình:

a+b=38570 và a=3b+922

=>a=29158 và b=9412

1: C=4a+2a+10b-b

=6a+9b

=3(2a+3b)

=3*12=36

D=21a+9b-6a-4b

=15a+5b

=5(3a+b)

=5*18=90

B=5a+7a-4b-8b

=12a-12b

=12(a-b)

=12*8=96