x=20     

Lí do = 0 nè:

\(\frac{x-39}{19}+\frac{x-49}{29}=\frac{x-5}{39}+\frac{x-69}{49}\)

=> \(\frac{x-39}{19}+\frac{x-49}{29}=0+\left(\frac{x-5}{39}+\frac{x-69}{49}\right)\)

=> \(\frac{x-39}{19}+\frac{x-49}{29}-\left(\frac{x-5}{39}+\frac{x-69}{49}\right)=0\)

=> \(\frac{x-39}{19}+\frac{x-49}{29}-\frac{x-5}{39}-\frac{x-69}{49}=0\)

9 tại vì taphai tim ssh 

bằng 9 đó bạn đúng 10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000%

Chữ số tận cùng là: 9 vì có 9 chữ số 9 tận cùng.

59 + 64 = 36 + 63 + 59 _Sai_

129 + 157 - 29 =129 - 29 + 157 _Đúng_

129 + 157 - 29 = 157 + 129 - 29 _Đúng_

4 +16 x 25 = 16 + 4 x 25 _Sai_

21 + 49 : 7 = 49 + 21 : 7 _Sai_

(36 + 45 ) : 9 = (45+36 ) : 9 _Đúng_

(Cái này tính nhẩm là ra thôi bạn ạ! Đúng cho mình 1 tick nhé bạn!)

sai

đúng

đúng

a,x/8=2/x

=>x²=16

=>x=-4;4

vậy x=-4;4

b,\(\frac{x-39}{19}+\frac{x-49}{29}=\frac{x-59}{39}+\frac{x-69}{49}\)

\(\Rightarrow\left(\frac{x-39}{19}+1\right)+\left(\frac{x-49}{29}+1\right)=\left(\frac{x-59}{39}+1\right)+\left(\frac{x-69}{49}+1\right)\)

\(\Rightarrow\frac{x-20}{19}+\frac{x-20}{29}=\frac{x-20}{39}+\frac{x-20}{49}\)

\(\Rightarrow\frac{x-20}{19}+\frac{x-20}{29}-\frac{x-20}{39}-\frac{x-20}{49}=0\)

\(\Rightarrow\left(x-20\right)\left(\frac{1}{19}+\frac{1}{29}-\frac{1}{39}-\frac{1}{49}\right)=0\)

vì \(\frac{1}{19}+\frac{1}{29}-\frac{1}{39}-\frac{1}{49}\ne0\Rightarrow x-20=0\Rightarrow x=20\)

vậy x=20

\(\frac{x}{8}=\frac{2}{x}\Rightarrow x^2=8.2\)

\(x^2=16\Rightarrow x^2=\left(\pm4\right)^2\Rightarrow x=4;x=-4\)

Học tốt

\(x=20\)\(nha\)

\(\frac{x-39}{19}+\frac{x-49}{29}=\frac{x-59}{39}+\frac{x-69}{49}\)

Công mỗi vế cho 2 ta được:

\(\frac{x-39}{19}+\frac{x-49}{29}+2=\frac{x-59}{39}+\frac{x-69}{49}+2\)

\(\frac{x-39}{19}+1+\frac{x-49}{29}+1=\frac{x-59}{39}+1+\frac{x-69}{49}+1\)

\(\frac{x-39}{19}+\frac{19}{19}+\frac{x-49}{29}+\frac{29}{29}=\frac{x-59}{39}+\frac{39}{39}+\frac{x-69}{49}+\frac{49}{49}\)

\(\frac{x-20}{19}+\frac{x-20}{29}=\frac{x-20}{39}+\frac{x-20}{49}\)

\(\frac{x-20}{19}+\frac{x-20}{29}-\frac{x-20}{39}-\frac{x-20}{49}=0\)

\(\left(x-20\right).\frac{1}{19}+\left(x-20\right).\frac{1}{29}-\left(x-20\right).\frac{1}{39}-\left(x-20\right).\frac{1}{49}=0\)

Đặt thừa số chung (x-20) ra ngoài

\(\left(x-20\right).\left(\frac{1}{19}+\frac{1}{29}-\frac{1}{39}-\frac{1}{49}\right)=0\)

\(\text{Vì }\frac{1}{19}+\frac{1}{29}-\frac{1}{39}-\frac{1}{49}\ne0\text{ nên }x-20=0\Rightarrow x=20\)

a)15/16:3/8x3/4

=15/16:6/16x12/16

=15/16x16/6x12/16

=15/6x12/16

=30/16=15/8

b)5/11x18/29-5/11x8/29+5/11x19/29

=5/11x(18/29-8/29+19/29)

=5/11x1=5/11

c)189/30-(59/30-3/5)

=189/30-59/30+3/5

=189/30-59/30+18/30

=148/30=74/15