de phai la x.(x-1)(x+4)(x+5)=84 moi dung
pt<=>[x.(x+4)].[(x-1)(x+5)]=84
<=>(x2+4x)(x2+4x-5)=84
Dat x^2+4x=a ta dc : a.(a-5)=84
=> a2-5a-84=0
=> a2+7a-12a-84=0
=>a(a+7)-12(a+7)=0
=>(a-12)(a+7)=0
=> a=12 hoac a=-7
Voi a=12 thi x2+4x=12
=>x2+4x-12=0
=>(x-2)(x+6)=0
=> a=2 hoac a=-6
Voi a=-7 thi x2+4x=-7
=>x2+4x+7=0
=>(x2+4x+4)+3=0
=>(x+2)2+3≥3>0
=> pt vo nghiem
Neu thay dung thi cho minh 1 tick nhe