a: \(A=t^2\left(m+5\right)-t\left(m+5\right)\cdot t+\frac32+\left(t-m\right)\)
\(=t^2\cdot\left(m+5\right)-t^2\cdot\left(m+5\right)+\frac32+\left(t-m\right)\)
\(=t-m+\frac32\)
b: Khi m=-1 thì \(A=t+1+\frac32=t+\frac52\)
Đặt A=0
=>\(t+\frac52=0\)
=>\(t=-\frac52\)
