Envision Math 2.0: Grade 8, Volume 1
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4. Evaluate Square Roots and Cube Roots
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Exercise 18 Page 30

Practice makes perfect

We consider a toy which has various shaped objects that a child can push through matching holes. We will consider a square hole and a cube-shaped block. We know the area of the square hole, so we can find its edge length.

Which edge length? Can it be found?
Square hole Yes

The block we consider is cube-shaped and we know its volume of 64 cubic centimeters. Therefore, it enables us to obtain edge length of the block.

Which edge length? Can it be found?
Square hole Yes
Cube-shaped block Yes

Edge Length of the Hole

Let's first find the edge length of the square hole. We know the hole is a square with an area of 8 square centimeters. Recall that a square has four equal sides. Let's call the side length s.
Square chip
When we find the area of a square, we multiply its side measurements two times. A = s * s ⇒ A= s^2 We know that the area A is equal to 8 square centimeters. Therefore, we can substitute this value into the equation for the area.
A=s^2
8=s^2
s^2 = 8
s= sqrt(8)
We know that the edge length of the hole is equal to sqrt(8) centimeters.

Edge Length of the Block

We also know the volume of the cube-shaped block of 64 cubic centimeters. This enables us to calculate the edge length of the block. Recall that three side measurements of a cube — the length, the width, and the height — all happen to be the same. This time let's call the side length l.

Cube block
We can find the volume of a cube by multiplying its side length three times. V= l* l* l Recall that multiplying a number by itself three times is equal to raising the number to the power of 3. V= l* l* l ⇒ V= l^3 We know that the volume of a cube-shaped block is 64 cubic centimeters. Therefore, we can substitute it into the equation for the volume.
V=l^3
64=l^3
l^3 = 64
We need to find a number that, when cubed, is equal to 64.
l^3 = 64
sqrt(l^3)=sqrt(64)
sqrt(l^3)=sqrt(4 * 4 * 4)
sqrt(l^3)=sqrt(4^3)
l= 4
We know that the edge length of the block is 4 centimeters.
We want to find out if the block will fit in the square hole. If the edge length of the block appears to be smaller than of equal to the edge length of the hole, then the block will fit in the hole. Therefore, we need to compare the hole edge length with the block edge length. c|c Hole Edge Length& Block Edge Length sqrt(8) & 4We want to find out whether 4 is greater than, less than, or equal to sqrt(8). To do it, we will estimate sqrt(8) to the whole number. Let's narrow down our estimate by looking at nearby perfect cubes. The two nearest perfect cubes are 4 and 9.
4<8<9
sqrt(4)< sqrt(8)
sqrt(2 * 2)< sqrt(8)
sqrt(2^2)< sqrt(8)
2< sqrt(8)<3
Therefore, we see that the hole edge length is less than 3, so it is also smaller than the block edge length of 4. sqrt(8)<3< 4 sqrt(8) < 4 This means the block will not fit in the square hole.