a)`A=2x^2+3x=5`(mik k hiểu đề cho lắm tại thấy bn ghi `2x^2+3x=5` nên mik lm v sai thì xl)
`⇔2x^2+3x-5=0`
`⇔2x^2+5x-2x-5=0`
`⇔(2x+5)(x-1)=0`
\(\left[ \begin{array}{l}2x+5=0\\x-1=0\end{array} \right.\)
\(\left[ \begin{array}{l}x=-5/2\\x=1\end{array} \right.\)
`⇒`Vậy `S={-5/2;1}`
a)`A.B=(2x^2+3x+5).(x^2-x+3)`
`=2x^4-2x^3+6x^2+3x^3-3x^2+9x+5x^2-5x+15`
`=2x^4+x^3+8x^2+4x+15`
b)Thay `x=2` ta có:
`=2x^4+x^3+8x^2+4x+15`
`=2.2^4+2.^3+8.2^2+4.2+15`
`=95`
