Maths

74 MATHEMATICS
(ii) The product of two consecutive positive integers is 306. We need to find the
integers.
(iii) Rohan’s mother is 26 years older than him. The product of their ages (in years)
3 years from now will be 360. We would like to find Rohan’s present age.
(iv) A train travels a distance of 480 km at a uniform speed. If the speed had been
8 km/h less, then it would have taken 3 hours more to cover the same distance. We
need to find the speed of the train.
4.3 Solution of a Quadratic Equation by Factorisation
Consider the quadratic equation 2x 2 – 3x + 1 = 0. If we replace x by 1 on the
LHS of this equation, we get (2 × 1 2 ) – (3 × 1) + 1 = 0 = RHS of the equation.
We say that 1 is a root of the quadratic equation 2x 2 – 3x + 1 = 0. This also means that
1 is a zero of the quadratic polynomial 2x 2 – 3x + 1.
In general, a real number is called a root of the quadratic equation
ax 2 2
+ bx + c = ✁ 0, a 0 if a + b + c = 0. We also say that x = is a solution of
the quadratic equation, or that satisfies the quadratic equation. Note that the
zeroes of the quadratic polynomial ax 2 + bx + c and the roots of the quadratic
equation ax 2 + bx + c = 0 are the same.
You have observed, in Chapter 2, that a quadratic polynomial can have at most
two zeroes. So, any quadratic equation can have atmost two roots.
You have learnt in Class IX, how to factorise quadratic polynomials by splitting
their middle terms. We shall use this knowledge for finding the roots of a quadratic
equation. Let us see how.
Example 3 : Find the roots of the equation 2x 2 – 5x + 3 = 0, by factorisation.
Solution : Let us first split the middle term – 5x as –2x –3x [because (–2x) × (–3x) =
6x 2 = (2x 2 ) × 3].
So, 2x 2 – 5x + 3 = 2x 2 – 2x – 3x + 3 = 2x (x – 1) –3(x – 1) = (2x – 3)(x – 1)
Now, 2x 2 – 5x + 3 = 0 can be rewritten as (2x – 3)(x – 1) = 0.
So, the values of x for which 2x 2 – 5x + 3 = 0 are the same for which (2x – 3)(x – 1) = 0,
i.e., either 2x – 3 = 0 or x – 1 = 0.
3
Now, 2x – 3 = 0 gives ✂ x and x – 1 = 0 gives x = 1.
2
3
So, ✂ x and x = 1 are the solutions of the equation.
2
In other words, 1 and 3 2 are the roots of the equation 2x2 – 5x + 3 = 0.
Verify that these are the roots of the given equation.