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# Core 1 maths - For what values of x does 9x^2 + 18x - 7 increase as x increases? Watch

1. For what values of x does 9x^2 + 18x - 7 increase as x increases?

The answer is x > -1

But I don't understand the question, I'v found out co-ordinates of stationary points in the previous part of the question and solved the equation. I'v got x=-1 for the stationary point, and I know this has an association with this.

Thanks
2. Well as it's a +ve quadratic function and the turning point is at -1 then surely when x < -1 the function is decreasing (going towards the stationary point) and when x > -1 the function is increasing.
3. The function increases when the gradient is positive. If we find dy/dx we get 18x + 18. The gradient of this is positive when it is > 0, so the inequality becomes 18x + 18 > 0. Hence 18x > -18, so x > -1.

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Updated: December 5, 2010
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