Question 173656
A perfect square would mean,
{{{4x^2 + 6.4x +k=(ax+b)^2}}}
If we expand the right hand side, we get,
{{{(ax+b)^2=a^2*x^2+(2ab)x+b^2}}}
Let's compare coefficients with your equation,
1.{{{a^2=4}}}
2.{{{2ab=6.4}}}
3.{{{b^2=k}}}
From 1, a can be either 2 or -2.
Let's start with a=2 and find the others,
From 2,
{{{2ab=6.4}}}
{{{2(2)b=6.4}}}
{{{b=6.4/4=1.6}}}
From 3,
{{{b^2=k}}}
{{{k=(1.6)^2}}}
{{{k=2.56}}}

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We can also work with a=-2.
From 2,
{{{2ab=6.4}}}
{{{2(-2)b=6.4}}}
{{{b=6.4/(-4)=-1.6}}}
From 3,
{{{b^2=k}}}
{{{k=(-1.6)^2}}}
{{{k=2.56}}}
And we're led to the same answer.