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If the sum and product of the roots of a quadratic equation are 211тАЛ and 215тАЛ, respectively, then the equation whose roots are double the roots of the given equation is:
x 2 + 11x + 30 = 0
x 2 - 11x + 30 = 0
x 2 + 11x - 30 = 0
x 2 - 11x - 30 = 0 2024/12/21-21:01:21
x 2 - 11x + 30 = 0
If roots are multiplied by a factor k=2, the new sum is k├ЧS and the new product is k2├ЧP. Here, 2├Ч(11/2)=11 and 4├Ч(15/2)=30. The new equation is x2тИТ11x+30=0.
Sum of roots of the original equation = 11/2, Product of roots of the original equation = 15/2.
x2тИТ(╬▒+╬▓)x+╬▒╬▓=0
If roots are multiplied by a factor k=2, the new sum is k├ЧS and the new product is k2├ЧP. Here, 2├Ч(11/2)=11 and 4├Ч(15/2)=30. The new equation is x2тИТ11x+30=0.
Students often forget that when roots are multiplied by k, the product of the roots must be multiplied by k2 (not just k).
Identify roots
Let the original roots be ╬▒ and ╬▓. We are given ╬▒+╬▓=211тАЛ and ╬▒╬▓=215тАЛ.
╬▒+╬▓=211тАЛ,╬▒╬▓=215тАЛ
Determine new roots
The new equation has roots 2╬▒ and 2╬▓. The new sum of roots is 2╬▒+2╬▓=2(╬▒+╬▓)=2├Ч211тАЛ=11.
SumnewтАЛ=11
Calculate new product
The new product of roots is (2╬▒)(2╬▓)=4(╬▒╬▓)=4├Ч215тАЛ=30.
ProductnewтАЛ=30
Construct the equation
Using the standard form x2тИТ(SumnewтАЛ)x+ProductnewтАЛ=0, substitute the values calculated.
x2тИТ11x+30=0
B is correct because the new sum of roots is 11 and the new product of roots is 30, which fits the equation x2тИТ11x+30=0.
This concept is a specific case of the transformation of equations f(x)=0 to f(x/k)=0 where k is the scaling factor of the roots.