Question 1

Verify that the numbers given alongside of the cubic polynomials below are their zeroes. Also verify the relationship between the zeroes and the coefficients in each case:

Answer


(i)                         Here f(x) = 2x³ + x² - 5x + 2

                            Given roots of f(x) are ½, 1, -2

F(1/2) = 2×(1/2)³ + (1/2)² - 5(1/2 ) + 2 = 0

F(1) = 2(1)³ + 1² - 5(1) + 2 = 0

F(-2) = 2(-2)³ + (-2)² - 5(-2) + 2 = 0

Hence, ½, 1 and -2 are the zeroes of f(x).

Therefore, sum of zeroes = -b/a -1/2

Sum of product of zeroes taken two at a time = c/a = -5/2

Product of zeroes = -d/a = 2

(ii)                    Let the f(x) = ax³ + bx² + c + d

Let α, β and γ be the zeroes of the polynomial f(x).

Then, sum of zeroes = -b/a = 2/1 ………………(i)

Sum of product of zeroes taken two at a time = c/a = -7. ………………..(ii)

Product of zeroes = -d/a = -14 ……………….(iii)

From equation (i), (ii) and (iii) we have

a = 1 , b = -2 , c = -7 and d = 14

Therefore the required polynomial on putting the values of a, b, c and d

F(x) = x³ - 2x² - 7x + 14

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