Don’t fall for the SAT tricks and traps. Here is a Math problem that is not as complicated as it looks:
13. If x2 +y2 = 87 and xy = 38, what is the value of (x + y)2?
A) 87
B) 125
C) 163
D) 250
E) 361
Identify: Let’s identify the problem type. There are variables in an equation, and it looks like I will need to manipulate those variables somehow. This must be an ALGEBRA problem.
Set Up: Write down what you know, even if you have to rewrite the question in a different way.
x2 + y2 = 87
xy =38
Is there any way to substitute xy into the first equation? It looks like I can’t because the variables in the first equation are squared. Maybe they are related somehow? Let’s see what the problem is asking because that can give us a clue as to how to manipulate these equations.
(x + y)2?
Hey, I know, this expression can be expanded:
(x + y)2 = x2 + 2xy + y2
Make Sure: Make sure you are answering the right question and you haven’t missed a step. Regroup the terms.
(x + y)2 =( x2 + y2)+ 2xy
Execute: Now, just substitute the given equations:
(x + y)2 =87 + 2(38)
Solve arithmetic with a calculator.
(x + y)2 =163
Answer: C
[Reference: Test 6G, Section 7, Problem 13]
