To find the answer to "What two numbers have a product of -9 and a sum of 21?" we first state what we know. We are looking for x and y and we know that x • y = -9 and x + y = 21.
Before we keep going, it is important to know that x • y is the same as y • x and x + y is the same as y + x. The two variables are interchangeable. Which means that when we create one equation to solve the problem, we will have two answers.
To solve the problem, we take x + y = 21 and solve it for y to get y = 21 - x. Then, we replace y in x • y = -9 with 21 - x to get this:
x • (21 - x) = -9
Like we said above, the x and y are interchangeable, therefore the x in the equation above could also be y. The bottom line is that when we solved the equation we got two answers which are the two numbers that have a product of -9 and a sum of 21. The numbers are:
-0.42016
21.42016
That's it! The two numbers that have a product of -9 and a sum of 21 are -0.42016 and 21.42016 as proven below:
-0.42016 • 21.42016 ≈ -9
-0.42016 + 21.42016 = 21
Note: Answers are rounded up to the nearest five decimals if necessary so the answers may not be exact.
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