Câu 1:
`5x(x-2015)-x+2015=0`
`<=> 5x(x-2015)-(x-2015)=0`
`<=> (x-2015)(5x-1)=0`
`<=>`\(\left[ \begin{array}{l}x-2015=0\\5x-1=0\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=2015\\x=\dfrac{1}{5}\end{array} \right.\)
Câu 2:
`86^2+172.14+14^2`
`=86^2+2.86.14+14^2`
`=(86+14)^2`
`=100^2`
`=10000`
Câu 3:
`x^3+3x^2+5x+a`
`=x^3+3x^2+5x+15+a-15`
`=x^2(x+3)+5(x+3)+a-15`
`=(x+3)(x^2+5)+a-15`
Để `x^3+3x^2+5x+a` $\vdots$ `x+3`
`=> (x+3)(x^2+5)+a-15` $\vdots$ `x+3`
mà `(x+3)(x^2+5)` $\vdots$ `x+3`
`=> a-15` $\vdots$ `x+3`
`=> a-15=0`
`<=> a=15`
Vậy `a=15`
