# ''Incomplete'' expression

Algebra Level 4

In the expansion of $$(1 - x- kx^2)^8$$, the coefficient of $$x^3$$ is 3.5 times the sum of the coefficients of $$x$$ and $$x^2$$.

$$k$$ can be expressed as $$\frac{a}{b}$$ where a and b are coprime positive integers. What is the value of $$a+b$$?

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