Đáp án:

a)$\dfrac{x-1}{59}$+$\dfrac{x-2}{58}$+$\dfrac{x-3}{57}$=$\dfrac{x-4}{56}$+$\dfrac{x-5}{55}$+$\dfrac{x-6}{54}$

`<=>`$\dfrac{x-1}{59}$-1+$\dfrac{x-2}{58}$-1+$\dfrac{x-3}{57}$=$\dfrac{x-4}{56}$-1+$\dfrac{x-5}{55}$-1+$\dfrac{x-6}{54}$-1

`<=>`$\dfrac{x-60}{59}$+$\dfrac{x-60}{58}$+$\dfrac{x-60}{57}$=$\dfrac{x-60}{56}$+$\dfrac{x-60}{55}$+$\dfrac{x-60}{54}$

`<=>`$\dfrac{x-60}{59}$+$\dfrac{x-60}{58}$+$\dfrac{x-60}{57}$-$\dfrac{x-60}{56}$-$\dfrac{x-60}{55}$-$\dfrac{x-60}{54}$`=0`

`<=>(x-60)(1/59+1/58+1/57-1/56-1/55-1/54)=0`

`<=>x-60=0(vì  1/59+1/58+1/57-1/56-1/55-1/54>0)`

`<=>x=60`

Vậy `x=60`

b)$\dfrac{2x+19}{21}$-$\dfrac{2x+17}{23}$=$\dfrac{2x+7}{33}$-$\dfrac{2x+5}{35}$

`<=>`$\dfrac{2x+19}{21}$+1-$\dfrac{2x+17}{23}$+1=$\dfrac{2x+7}{33}$+1-$\dfrac{2x+5}{35}$+1

`<=>`$\dfrac{2x+40}{21}$-$\dfrac{2x+40}{23}$=$\dfrac{2x+40}{33}$-$\dfrac{2x+40}{35}$

`<=>`$\dfrac{2x+40}{21}$-$\dfrac{2x+40}{23}$-$\dfrac{2x+40}{33}$+$\dfrac{2x+40}{35}$`=0`

`<=>(2x+40)(1/21-1/23-1/33+1/35)=0`

`<=>2x+40=0(vì  1/21-1/23-1/33+1/35>0)`

`<=>2x=-40`

`<=>x=-20`

Vậy `x=-20`

c)`5^n+5^{n+2}=650`

`<=>5^n+5^n .5^2=650`

`<=>5^n(1+5^2)=650`

`<=>5^n.26=650`

`<=>5^n=25`

Mà `25=5^2`

`=>n=2`

Vậy `n=2`

`@`$karenn$

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