- #1
zebo
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Homework Statement
Disclaimer: English is not my first language, so i apologize for any wrong math-terms.
We look at the function f(x) = x^3. On the graph for f we have a point, P(a,a^3), where a =/= 0. The tangent to f through P cuts through f in another point, Q. Find Q and show, that the tangent to f through Q has a slope which is 4 times as steep as the tangent to f through P.
Homework Equations
The Attempt at a Solution
Tangent through P cuts the function f in Q, which means in Q tangentP=x^3.
3a^2(x-a)+a^3=x^3 <=>
3a^2=(x^3-a^3)/(x-a) <=>
3a^2=x^2-a^2 <=>
4a^2=x^2
2a=x
Which means Q is located in (2a,(2a)^3)<=> Q(2a,8a^3)
But this means that the tangent through Q has a slope which is twice as steep as it should be, since:
tangentQ=8a^3+24a^2(x-a)
I am not sure how i should solve this problem, and i hope you are able to understand my sloppy english/and math.