<p>If f(x)=x+5, for what value of x does f(4x)=f(x+4) ?
The explanation from the book gave me no clue what do I have to do.</p>
<p>This is a question from the Barron's 25th edition book from page 627. </p>
<p>Also, from your personal experience or from your opinion what kind of resources/ which books are the best for SAT Functions and the hard problems?
Thanks!</p>
<p>If you were asked for f(2), how would you find it? You’d see that f(x) is x+5 and replace x with 2, so you know that f(2)=2+5, which is 7.</p>
<p>In the same way, f(4x)=(4x)+5—in non-mathematical terms, you’re basically just replacing what’s inside the parentheses of the function and replacing ‘x’ with it in ‘x+5’. If f(x)=x+5, then we can also say that f(x+4)=(x+4)+5.</p>
<p>So, we now know f(4x) and f(x+4), and we’re asked to set them equal to each other and solve for x:
f(4x) = f(x+4)
4x+5 = x+4+5
4x = x+4
3x = 4
x = 4/3</p>
<p>Oh, thanks! Now I see it. It was not even tough at all haha</p>