Dipanjan005
IanStewart can you please explain how to solve this because i am not able to understand the explanations provided?
I wouldn't worry about this question much, because it's not like any real GMAT question. Really the things you'd want to know that we use in this question are:
- how to plug algebraic things into functions, so how to work out what f(x+1) is when a question tells you what f(x) is
- that |x| = |-x|, and that |x| = -x when x is negative
If you think about what the function looks like in coordinate geometry, you can evaluate items I and II quickly, so I'll describe that, but you can also just plug in numbers to see what happens. The function is in two parts:
• when x is positive (or zero, though zero doesn't really matter here), the function is the line y = 1 - x, so it has a y-intercept of 1, and a slope of -1.
• when x is negative, the function is the line y = x - 1, so it has a y-intercept of -1 and a slope of 1
We then need to see which of the three roman numeral items must be true. Absolute values are always positive (or zero), so looking at I:
F(|x|) = | F(x) |
here we're plugging a positive thing into the function on the left, and item I says we're always getting a (specific) positive result. But we know when we plug in positive things the function is a line with slope -1, so the function will take negative values for sure if we plug in something large enough, and this can't be right. You could also see that by plugging in x = 2, say; then F(|x|) = F(|2|) = F(2) = -1, which is not equal to |F(2)| = |1 - 2| = |-1| = 1.
Looking at II. if F(X) = F(-X), then we get the same result when we plug in a number or its negative. That would mean the graph is symmetric if we reflect the function through the y-axis, but that's not the case here, if you visualize the lines that make up the function. Or you can again plug in x = 2, and then F(X) = F(2) = -1, while F(-2) = x - 1 = -3, which aren't equal.
For III, when x is positive, |x| = x, and x+1 is positive too. So when x > 0, then |F(x+1)| = |x| just says, plugging "x+1" into the definition of the function for positive values, |1 - (x+1)| = x, or |-x| = x, and since |-x| = |x| = x when x is positive, this is true.
When x is negative, then |x| = -x. We have two cases now, when we plug into the function. Either x + 1 is also negative (x < -1) so we'll use the negative definition of the function, or x + 1 > 0, or -1 < x < 0, and we'll use the positive definition of the function. When x and x + 1 are both negative, item III becomes
|F(x + 1)| = |x|
|(x + 1) - 1| = -x
|x| = -x
and since x < 0, the left and right sides are equal. When x is negative and x+1 is positive, then using the positive definition of the function, we have
|F(x + 1)| = |x|
|1 - (x + 1)| = -x
|-x| = -x
|x| = -x
and again since x < 0, these are equal. So III is true in every case. I've never seen an official question like this one though.