VeritasPrepKarishma wrote:
goodyear2013 wrote:
The sides of rectangle X are each multiplied by a to form rectangle Y and by b to form rectangle Z. a times the area of X is 10, and b times the area of X is 5. If the difference in area between Y and Z is 300, what is a - b?
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Set sides of rectangle X are L and W
Area of X: = LW Area of Y = (aL)(aW) = a^2 * LW Area of Z = (bL)(bW) = b^2 * LW a times area X = aLW = 10 b times area X = bLW = 5
Which means that difference between Y and Z = aaLW – bbLW = 300.
And since you're looking to solve for a - b, you can try to get a and b alone by factoring out the common LW terms:
LW(a^2 - b^2) = 300
Which gives you the Difference of Squares setup that allows you to get (a - b) alone:
LW(a + b)(a - b) = 300
Then you should see that if you distribute LW across the first set of parentheses, you can get aLW and bLW, for which you have actual values:
(aLW + bLW)(a - b) = 300
(10 + 5)(a - b) = 300 15(a - b) = 300 (a - b) = 20
Hi, I want to know if we have the simpler solution, please
Assume the side of rectangle X is s.
X (side s, area s^2)
Y (side as, area (as)^2)
Z (side bs, area (bs)^2)
Given: \(as^2 = 10\), \(bs^2 = 5\) ..........(I)
\((as)^2 - (bs)^2 = 300\)
\(s^2 (a+b)(a-b) = 300\) ...........(II)
We need to find (a-b).
From (I), \(as^2 + bs^2 = 15 = s^2(a+b)\) (you do this because you need to get rid of s^2 and (a+b) in equation (II) above)
Substitute this in (II) to get \(15*(a-b) = 300\)
(a-b) = 20
Another way of doing this..
Let the original sides be x and y ---- area = \(xy\)
For Rectangle X -- ax and ay ---- area = \(a^2xy\)
For Rectangle Y -- bx an by ---- area = \(b^2xy\)
Now, it's given that
\(axy = 10\)
Thus
\(a = \frac{10}{(xy)}\)
Similarly we can write for Z as
\(bxy = 5\)
Thus
\(b = \frac{5}{(xy)}\)
It can clearly be seen that
\(a = 2b\)
Substituting "axy" in the original area equation..we can see that the area of X can be written as = 10a
Similarly for B it can be written as = 5b
Difference in areas
\(10a - 5b = 300\)
Using value of a as 2b, we get
b = 20
a = 40
Thus
a-b = 20
(B)