What is the value of r^2 - 2rs + s^2?

Dear ohmr,

I'm happy to respond.

My friend, you are relatively new to GMAT Club. When there's something you don't understand about a question, it's much better to post a reply and ask your question. Opening a report only appropriate if you have evidence that the question is wrong (for example, if you are familiar with the book or website where the question appears, and the way it appears here is different from the source). If it's just your own opinion that the answer is different, please discuss that in the thread itself--don't post a report.

My friend you are confused about a subtlety that confuses many people---the difference between an algebraic expression vs. an algebraic equation.
I. an algebraic expression: \(x^2 - 4x - 21\)
The variable x could take on any value on the number line, and for different values of x, this expression would take different values. In fact, we could set y equation to this expression and get the graph of a parabola.
II. an algebraic equation: \(x^2 - 4x - 21 = 0\)
Unlike (I), this is a complete mathematical sentence. On the entire number line, x = -3 and x = +7 are the only two values that satisfy the equation. These are the two values that solve the equation.

Notice that we can solve an equation---we can solve an expression!

This DS is about finding the value of an expression. It gives us an expression with two variables, and asks us to find the value of this expression.

You were treating it as if were about solving an equation, rather than evaluating an expression.

Does this distinction make sense?
Mike

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