The cubic equation x³ +bx² +cx+d = 0, where b, c and d are constants, has roots α, β and γ. It is given that α+β + γ = 2, α² + β² + γ² = 3, α⁴ + β⁴ + γ⁴ = 5.
- (a)Find the values of b and c.[3]
- (b)Find the value of d.[5]
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2025). The mark or mark type is shown in brackets.
- [B1] b = -(α + β + γ) = -2
- [M1] Uses formula for sum of squares. (3 = (-2)² - 2c)
- [A1] c = 1/2
- [M1] Uses original equation or formula for sum of cubes. (S3+bS2+cS₁ + 3d = 0 OR S3 = -3b-2c-3d = 5-3d)
- [A1FT] Allow FT from part (a) for their value.
- [M1] Uses original equation. (S4+bS3+cS2+dS₁ = 0)
- [A1FT] Allow FT from part (a) for their value and for their value of S3.
- [A1] CAO (d = 7/16)
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