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.
Please help me understand what the question is asking.
Core 1 maths - For what values of x does 9x^2 + 18x - 7 increase as x increases? Watch
- Thread Starter
- 05-12-2010 15:01
- 05-12-2010 15:05
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.
- 05-12-2010 15:08
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.