As can be seen, this formula can be used when there is a multiplication between the sum of two particular elements and the subtraction between the two elements. Instead of presenting them as a multiplication of sum and subtraction, it can be written X2−Y2 and it expresses exactly the same thing. In the same way, if such an expression X2−Y2 representing the subtraction of two squared numbers is presented to you, you can write it like this: (X+Y)×(X−Y) Pay attention: the formula works both in non-algebraic expressions and in expressions that combine unknowns and numbers.
If we are given: (x+4)(x−4) We can see that we are referring to a multiplication between the sum of two elements and the difference between them. Therefore, we can present the same expression according to the formula in the following way: x2−42 x2−16 In the same way, if we were given the expression: x2−16 We could express 16 as a squared number, that is 42, Obtain a representation that fits the formula: x2−42 From here using the formula and presenting the expression in the following way:
x2−42=(X−4)(x+4)
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Examples and exercises with solutions for multiplying the sum of two elements by the difference between them
Exercise #1
Solve:
(2+x)(2−x)=0
Video Solution
Step-by-Step Solution
We use the abbreviated multiplication formula:
4−x2=0
We isolate the terms and extract the root:
4=x2
x=4
x=±2
Answer
±2
Exercise #2
Solve the following equation:
x2+10x+50=−4x+1
Video Solution
Step-by-Step Solution
The equation in the problem is:
x2+10x+50=−4x+1First, we identify that the equation is quadratic (and this is because the quadratic term in it does not cancel out), therefore, we will simplify the equation by moving all terms to one side and combine thelike terms:
x2+10x+50=−4x+1x2+10x+4x+50−1=0x2+14x+49=0
We want to solve this equation using factorization.
First, we'll check if we can find a common factor, but this is not possible, since there is no multiplicative factor common to all three terms on the left side of the equation.
We can factor the expression on the left side using the quadratic factoring formula for a trinomial, however, we prefer to factor it using the trinomial factoring methodl:
Note that the coefficient of the quadratic term (the term with the second power) is 1, and therefore we can try to perform factoring according to the quick trinomial method:
But before we do this in the problem - let's remember the general rule for factoring with thequick trinomial method:
The rule states that for the algebraic quadratic expression:
x2+bx+cWe can find a factorization to the form of a product if we can find two numbers m,nsuch that the conditions (conditions of the quick trinomial method) are met:
{m⋅n=cm+n=bIf we can find two such numbers m,nthen we can factor the general expression mentioned above into the form of a product and present it as:
x2+bx+c↓(x+m)(x+n)which is its factored form (product factors) of the expression,
Let's return now to the equation in the problem that we received in the last stage after arranging it:
x2+14x+49=0Note that the coefficients from the general form we mentioned in the rule above:
x2+bx+care:{c=49b=14Don't forget to consider the coefficient together with its sign.
Let's continue - we want to factor the expression on the left side into factors according to the quick trinomial method, above, so we'll look for a pair of numbers m,n that satisfy:
{m⋅n=49m+n=14We'll try to identify this pair of numbers using our knowledge of the multiplication table, we'll start from the multiplication between the two required numbers m,n that is - from the first row of the pair of requirements we mentioned in the last stage:
m⋅n=49We identify that their product needs to give a positive result, and therefore we can conclude that their signs are identical.
Next, we'll refer to the factors (integers) of the number 49, and from our knowledge of the multiplication table we can know that there are only two possibilities for such factors: 7 and 7, or 49 and 1, as we previously concluded that their signs must be identical, a quick check of the two possibilities for the second condition:
m+n=14 will lead to a quick conclusion that the only possibility for fulfilling both of the above conditions together is:
7,7That is:
m=7,n=7(It doesn't matter which one we call m and which one we call n)
It is satisfied that:
{7⋅7=497+7=14 From here - we understood what the numbers we are looking for are and therefore we can factor the expression on the left side of the equation in question and present it as a product:
x2+14x+49↓(x+7)(x+7)
In other words, we performed:
x2+bx+c↓(x+m)(x+n)
If so we factored the quadratic expression on the left side of the equation into factors using factoring according to the quick trinomial method, and the equation is:
x2+14x+49=0↓(x+7)(x+7)=0(x+7)2=0In the last stage we notice that the expression on the left side the term:
(x+7)
is multiplied by itself and therefore the expression can be written as a squared term:
(x+7)2
Now that the expression on the left side has been factored into a product form (in this case not just a product but actually a power form) we will continue to the quick solution of the equation we received:
(x+7)2=0
Let's pay attention to a simple fact, on the left side there is a term that is raised to the second power, and on the right side the number 0.
0 squared (to the second power) will give the result 0, so we get that the equation equivalent to this equation is the equation:
x+7=0(We could have solved algebraically and taken the square root of both sides of the equation, we'll discuss this in a note at the end)
We'll solve this equation by transferring the constant number to the other side and we'll get that the only solution is:
x=−7Let's summarize then the stages of solving the quadratic equation using the quick trinomial factoring method:
x2+14x+49=0↓(x+7)(x+7)=0(x+7)2=0↓x+7=0x=−7Therefore, the correct answer is answer B.
Note:
We could have reached the final equation by taking the square root of both sides of the equation, however - taking a square root involves considering two possibilities: positive and negative (it's enough to consider this only on one side, as described in the calculation below), that is, we could have performed:
(x+7)2=0/↓(x+7)2=±0x+7=±0x+7=0
On the left side, the root (which is a half power) and the second power canceled each other out, and on the right side the root of 0 is 0, and we considered two possibilities positive and negative (this is the plus-minus sign indicated) except that the sign (which is actually multiplication by one or minus one) does not affect 0 which remains 0 in both cases, and therefore we reached the same equation we reached by logic - in the solution above.
In a case where on the right side there's a number other than 0, we could solve only by taking the root and considering the two positive and negative possibilities which would then give two different possibilities for the solution.
Answer
x=−7
Exercise #3
(2x)2−3=6
Video Solution
Step-by-Step Solution
First we rearrange the equation and set it to 0
4x2−3−6=0
4x2−9=0
We then apply the shortcut multiplication formula:
4(x2−49)=0
x2−(23)2=0
(x−23)(x+23)=0
x=±23
Answer
±23
Exercise #4
Complete the following exercise:
(x+21)(x−21)=0
Video Solution
Answer
41
Exercise #5
Solve the exercise:
(x+3)(x−3)+(x+1)(x−1)=0
Video Solution
Answer
±5
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