Answer only one of the following alternatives. EITHER The point A(0, 10) lies on the curve for which dy/dx = e⁻ˣ/⁴. The point B, with x-coordinate –4, also lies on the curve. (i) Find, in terms of e, the y-coordinate of B. (ii) The tangents to the curve at the points A and B intersect at the point C. Find, in terms of e, the x-coordinate of the point C.
- (i)Find, in terms of e, the y-coordinate of B.[5]
- (ii)Find, in terms of e, the x-coordinate of the point C.[5]
Mark scheme
Marking points from the official mark scheme (Oct/Nov 2012). The mark or mark type is shown in brackets.
- [M1] Integrate to find y
- [A1] y = -4e⁻ˣ/⁴ (+ c)
- [DM1] Substitute (0, 10)
- [A1] c = 14
- [A1] y = 14 - 4e⁻ˣ/⁴
- [A1] 14 - 4e
- [B1] Tangent at A is y – 10 = x
- [B1] Gradient tangent at B is e
- [B1] Tangent at B is y + 4e - 14 = ex + 4e
- [M1] Solve equations of tangents
- [A1] x = 4 / (1 - e) o.e.
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