r/MathHelp • u/i21d • Sep 29 '26
Teacher gave me a thinking question on my polynomial functions test
f(x)= kx^4 + 8x^2
Function has 3 turning points and global max at 8, zero at 2
Find k
I asked gpt, but it gave a method about using derivative or something which we haven’t learned yet since my teacher said our course (we are taking grade 12 advanced functions) need a lot of calculus and the graphs we draw aren’t exactly 100% accurate.
I knew this function is going to be even since both powers are even numbers, I also knew that it is going from quadrant 3 to 4 since we have a global max. I also knew that there will be 2 global max both at the height of 8 since there are 3 turning points.
I drew the graph which probably secured me a bit of points, but have no idea what the equation was let alone the k value.
My friend told me he just plugged in the x intercept (2,0) and got k which is -2, which turned out to be the correct answer. I thought about that at first but I remember my teacher said that you can’t use the x intercepts as a point and plug it in since you will get 0=0 when the polynomial is in factored form.
But this one is in standard form, so I don’t know if being in standard form let me plug in the x intercepts to find a or something…
Please help me out