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16 votes
16 votes
Solve for w, where w is a real number.

Solve for w, where w is a real number.-example-1
User Ali Arslan
by
2.6k points

2 Answers

18 votes
18 votes

Answer: w=9.

Explanation:


√(-4w+61)=w-4

Tolerance range:


\left \{ {{-4w+61\geq 0} \atop {w-4\geq 0}} \right. \ \ \ \ \left \{ {:4 \atop {w\geq 4}} \right. \ \ \ \ \left \{ {{w\leq 15,25} \atop {w\geq 4}} \right. \ \ \ \ \Rightarrow\ \ \ \ \\w\in[4;15,25].

Solution:


(√(-4w+61))^2=(w-4)^2\\-4w+61=w^2-2*w*4+4^2\\-4w+61=w^2-8w+16\\w^2-4w-45=0\\D=(-4)^2-4*1*(-45)\\D=16+4*45\\D=16+180\\D=196.\\√(D)=√(196)\\√(D)=14 \\w_(1,2)=(-(-4)б14)/(2) \\w_1=(4-14)/(2) \\w_1=(-10)/(2) \\w_1=-5\ \\otin tolerance\ range.\\w_2=(4+14)/(2)\\ w_2=(18)/(2) \\w_2=9\ \in tolerance \ range.

User TheGtknerd
by
3.0k points
19 votes
19 votes

Answer:

w= 9

Explanation:


√( - 4w + 61) = w - 4

Square both sides:

-4w +61= (w -4)²


\boxed{(a - b)^(2) = a^2 -2ab + b^2 }

Expand:

-4w +61= w² -2(w)(4) +4²

-4w +61= w² -8w +16

Simplify:

w² -8w +16 +4w -61= 0

w² -4w -45= 0

Factorize:

(w -9)(w +5)= 0

w -9= 0 or w +5= 0

w= 9 or w= -5 (reject)

Note:

-5 is rejected since we are only taking the positive value of the square root here. If the negative square root is taken into consideration, then w= -5 would give us -9 on both sides of the equation.

Why do we use negative square root?

When solving an equation such as x²= 4,

we have to consider than squaring any number removes the negative sign i.e., the result of a squared number is always positive.

In the case of x²= 4, x can be 2 or -2. Thus, whenever we introduce a square root, a '±' must be used.

However, back to our question, we did not introduce the square root so we should not consider the negative square root value.

User Krzysztof Wolny
by
2.7k points
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