<p>which of the following gives only values of x for which the graph y= 4x3-7x2-45x+36 is decreasing?
answer:- -1.3 less than or equal x greater than or equal 3.3</p>
<p>Can you tell me the other answer choices.</p>
<p>One solution is to simply graph the function on your calculator.</p>
<p>If you forgot your calculator or otherwise don’t want to use it, an alternate solution is to find the derivative: dy/dx = 12x^2 - 14x - 45. This should have zeros at -1.3 and 3.3. Then dy/dx is negative when x is between these two numbers.</p>
<p>Im sry idk</p>
<p>MITer94,
I solved the equation without a calculator, and got 3,4,-0.75 as x-coordinates where y is zero. Where have I gone wrong?</p>
<p>MITer94,
I solved the equation without a calculator, and got 3,4,-0.75 as x-coordinates where y is zero. Where have I gone wrong?</p>
<p>You want the x-coordinates where dy/dx is zero, not where y = 0. The correct polynomial to solve is 12x^2 - 14x - 45 = 0.</p>
<p>However, SAT problems never require the use of calculus. Graphing the cubic would probably be faster.</p>
<p>Interestingly, the roots of the above equation are about -1.4 and 2.6. I graphed the original function and got the same result. Is your answer key correct?</p>
<p>The problem is I don’t own a graphic calculator. And umm, why do we solve dy/dx=0? Won’t it just give us the stationary points of the graph?</p>
<p>Yes, but we know that dy/dx = 12x^2 - 14x - 45 is a quadratic that points up. We solve for the critical points (dy/dx = 0) to obtain x = -1.4 and 2.6. Then any value of x between these two zeros will result in dy/dx being negative, i.e. y decreasing.</p>
<p>Oh now I get it. Thanks a ton! And by any chance do you have physics and math II satii ebooks?</p>
<p>Sorry, I don’t :(</p>