\(-5x^2+16x-3=-5x^2+15x+x-3=-5x\left(x-3\right)+\left(x-3\right)=\left(x-3\right)\left(-5x+1\right)\)
Phần 1 ) Đánh lộn dấu " ) " thành số 0 rồi bạn ....
\(\left(x+1\right).\left(x+2\right).\left(x+3\right).\left(x+4\right)-15\)
\(=\left[\left(x+1\right).\left(x+4\right)\right].\left[\left(x+2\right).\left(x+3\right)\right]-15\)
\(=\left(x^2+5x+4\right).\left(x^2+5x+6\right)-15\)
\(\text{Đặt }k=x^2+5x+4,\text{ta có: }\)
\(\left(x^2+5x+4\right).\left(x^2+5x+6\right)-15=k.\left(k+2\right)-15=k^2+2k-15=k^2-5k+3k-15=k.\left(k-5\right)+3.\left(k-5\right)=\left(k-5\right).\left(k+3\right)\)
tự thay vô nha : )
