# Past Papers’ Solutions | Edexcel | AS & A level | Mathematics | Core Mathematics 1 (C1-6663/01) | Year 2011 | June | Q#7

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Question where k is a real constant.

a.   Find the discriminant of f(x) in terms of k.

b.   Show that the discriminant of f (x) can be expressed in the form (k + a)2 + b, where a and b are  integers to be found.

c.   Show that, for all values of k, the equation f (x) = 0 has real roots.

Solution

a.

We are given; It is evident that we are given a quadratic equation and we are required to find discriminant of this  equation.

For a quadratic equation , the expression for solution is; Where is called discriminant.

Therefore;     b.

We have found discriminant of given quadratic equation in (a) as; We have the expression; We use method of “completing square” to obtain the desired form. Next we complete the square for the terms which involve .

We have the algebraic formula;  For the given case we can compare the given terms with the formula as below;  Therefore we can deduce that; Hence we can write; To complete the square we can add and subtract the deduced value of ;      c.

We are given that f(x)= has real roots.

Since ; For a quadratic equation , the expression for solution is; Where is called discriminant.

If , the equation will have two distinct roots.

If , the equation will have two identical/repeated roots.

If , the equation will have no roots.

Since we are given only equation has real roots but not that these roots are distinct or otherwise.

Therefore, for this equation; We have found in (a) that discriminant of this equation is; Therefore, we need to solve the inequality; We have also demonstrated in (b) that can be written as; Hence; It is evident that for all real values of k the above equation will be; Hence, the given equation will have real roots for all real values of k.