Find the values of k for which the line y = kx-7 and the curve y = 3x² +8x+5 do not intersect.
- Find the values of k for which the line y = kx-7 and the curve y = 3x² +8x+5 do not intersect.[6]
Mark scheme
Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.
- [M1] 3x²+8x+5= kx-7
- [A1] 3x² + (8-k) x + 12 [= 0] soi
- [M1] (8-k)²-4 (3) (12)
- [M1] k²-16k-80 < 0
- [A1] Critical values: -4 and 20 soi
- [A1] -4 < k < 20
- Alternative method: [M1] for k = 6x + 8 oe
- [M1] for y = (6x + 8)x-7
- [M1] for 3x² + 8x + 5 = (6x + 8)x−7
- [A1] for x = ±2
- [A1] for k=-4, k = 20
- [A1] for -4<k<20
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