Question : If $x^2+ax+b$ is a perfect square, then which one of the following relations between $a$ and $b$ is true?
Option 1: $a^2=b$
Option 2: $a^2=4b$
Option 3: $b^2=4a$
Option 4: $b^2=a$
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Correct Answer: $a^2=4b$
Solution :
Given:
$x^2+ax+b$ is a perfect square.
So, $x^2+2×x×\frac{a}{2}+(\sqrt{b})^2$
Now, the above quadratic polynomial will be a perfect square only if the coefficient of $x$ is $\pm 2$ times the square root of the product of the coefficient of $x^2$ and constant as follows.
$a=\pm 2 \sqrt{1×b}=\pm 2 \sqrt{b}$
$⇒a^2= (\pm 2 \sqrt{b})^2$
$\therefore a^2=4b$
Hence, the correct answer is $a^2=4b$.
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