PSAT Math : How to multiply negative numbers

Study concepts, example questions & explanations for PSAT Math

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Example Questions

Example Question #2 : How To Add / Subtract / Multiply / Divide Negative Numbers

If  is a positive number, and  is also a positive number, what is a possible value for ?

Possible Answers:

Correct answer:

Explanation:

Because \dpi{100} \small -3b is positive, \dpi{100} \small b must be negative since the product of two negative numbers is positive.

Because \dpi{100} \small ab is also positive, \dpi{100} \small a must also be negative in order to produce a prositive product.

To check you answer, you can try plugging in any negative number for \dpi{100} \small a.

Example Question #1 : How To Multiply Negative Numbers

, and  are all negative odd integers. Which of the following three expressions must be positive?

I) 

II) 

III) 

Possible Answers:

None of these

II only

All of these

III only

I only

Correct answer:

All of these

Explanation:

A negative integer raised to an integer power is positive if and only if the absolute value of the exponent is even. Since the sum or difference of two odd integers is always an even integer, this is the case in all three expressions. The correct response is all of these.

Example Question #2 : How To Multiply Negative Numbers

 is a positive integer;  and  are negative integers. Which of the following three expressions must be negative?

 

I) 

II) 

III) 

Possible Answers:

II and III only

None of I, II or III

I and III only

I, II and III

I and II only

Correct answer:

None of I, II or III

Explanation:

A negative integer raised to an integer power is positive if and only if the absolute value of the exponent is even; it is negative if and only if the absolute value iof the exponent is odd. Therefore, all three expressions have signs that are dependent on the odd/even parity of  and , which are not given in the problem. 

The correct response is none of these.

Example Question #3 : How To Multiply Negative Numbers

, and  are all negative numbers. Which of the following must be positive?

Possible Answers:

Correct answer:

Explanation:

The key is knowing that a negative number raised to an odd power yields a negative result, and that a negative number raised to an even power yields a positive result.

 and  are positive, yielding a positive dividend;  is a negative divisor; this result is negative.

:  and  are negative, yielding a positive dividend;  is a negative divisor; this result is negative.

 :  is positive and  is negative, yielding a negative dividend;  is a positive divisor; this result is negative.

 :  is negative and  is positive, yielding a negative dividend;  is a positive divisor; this result is negative.

  is positive and  is negative, yielding a negative dividend;  is a negative divisor; this result is positive.

The correct choice is .

Example Question #3 : Negative Numbers

 and  are positive numbers;  is a negative number. All of the following must be positive except:

Possible Answers:

Correct answer:

Explanation:

Since  and  are positive, all powers of  and  will be positive; also, in each of the expressions, the powers of  and  are being added. The clue to look for is the power of  and the sign before it.

In the cases of  and , since the negative number  is being raised to an even power, each expression amounts to the sum of three positive numbers, which is positive.

In the cases of  and , since the negative number  is being raised to an odd power, the middle power is negative - but since it is being subtracted, it is the same as if a positive number is being added. Therefore, each is essentially the sum of three positive numbers, which, again, is positive.

In the case of , however, since the negative number  is being raised to an odd power, the middle power is again negative. This time, it is basically the same as subtracting a positive number. As can be seen in this example, it is possible to have this be equal to a negative number:

:

Therefore,  is the correct choice.

Example Question #4 : Negative Numbers

Let  be a negative integer and  be a nonzero integer. Which of the following must be negative regardless of whether  is positive or negative?

Possible Answers:

None of the other answers is correct.

Correct answer:

Explanation:

Since  is positive, , the product of a negative number and a positive number, must be negative also.

Of the others:

 is incorrect; if  is negative, then  is positive, and  assumes the sign of .

 is incorrect; again,  is positive, and if  is a positive number,  is positive.

 is incorrect; regardless of the sign of  is positive, and if its absolute value is greater than that of  is positive.

Example Question #3 : How To Multiply Negative Numbers

Given that  are both integers,  , and , which of the following is correct about the sign of the expression  ?

Possible Answers:

The expression must be negative or zero.

The expression must be positive or zero.

The expression must be positive.

The expression can be positive, negative, or zero.

The expression must be negative.

Correct answer:

The expression must be negative or zero.

Explanation:

If , then we know that  is any number between or equal to  and . Therefore  must be a negative number. 

Also, if , then we know that  is any number between or equal to  and . Therefore  must be a negative number.

Now looking the expression  we can find the sign of each component in the expression.

Since  is negative, we know that a negative number minus another number is still a negative number.

Therefore,  is a negative number.

Since  is between or equal to   and  we can plug in these end values in to determine the sign of .

Therefore,  is either zero or a positive number.

 

Now to find the sign of the expression we look at the product of the two components. The product of a negative number and a positive number is a negative number; the product of a negative number and zero is zero. Therefore, the correct choice is that  is negative or zero.

 

Example Question #4 : How To Multiply Negative Numbers

Find the product.

Possible Answers:

Correct answer:

Explanation:

When multiplying together two negatives, our value for the product become positive. 

Example Question #3 : How To Multiply Negative Numbers

Find the product.

Possible Answers:

Correct answer:

Explanation:

Since we have one positive and one negative multiple, the resulting product must be negative. 

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