Thay x =\(\frac{1}{2}\)vào đa thức M(x) ta có :
M(x) = a . \(\left(\frac{1}{2}\right)^2\) + 5 . \(\frac{1}{2}\) - 3
=> M(x) = a . \(\frac{1}{4}\)- \(\frac{1}{2}\)
=> M(x) = \(\frac{1}{4}\)a - \(\frac{1}{2}\)
Cho \(\frac{1}{4}\)a - \(\frac{1}{2}\) = 0
=> \(\frac{1}{4}\)a = \(\frac{1}{2}\)
=> a = 2
Vậy hệ số a = 2.