a) `3/(x-4)=(x+4)/(3)` `(xne4)`
⇔`3.3=(x-4)(x+4)`
⇔`9=x^2+4x-4x-16`
⇔`9+16=x^2`
⇔`25=x^2`
⇔`x^2=+-5` `(tm)`
Vậy `x=+-5`
b) `(x+2)/(x+6)=3/(x+1)` `(xne-6,xne-1)`
⇔`(x+2)(x+1)=3(x+6)`
⇔`x^2+x+2x+2=3x+18`
⇔`x^2+3x-3x=18-2`
⇔`x^2=16`
⇔`x=+-4` `(tm)`
Vậy `x=+-4`