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8. If [tex]$x + y = 4$[/tex] and [tex]$xy = 5$[/tex], find the value of [tex][tex]$x^3 + y^3$[/tex][/tex].

Sagot :

Certainly! We are given the equations:

[tex]\[ x + y = 4 \][/tex]
[tex]\[ xy = 5 \][/tex]

We need to find the value of [tex]\( x^3 + y^3 \)[/tex].

To solve for [tex]\( x^3 + y^3 \)[/tex], we can use the identity involving the sum and product of cubes for two variables [tex]\( x \)[/tex] and [tex]\( y \)[/tex]:

[tex]\[ x^3 + y^3 = (x + y)^3 - 3xy(x + y) \][/tex]

Now, let's break this down step-by-step:

1. Calculate [tex]\( (x + y)^3 \)[/tex]:
[tex]\[ (x + y)^3 = (4)^3 = 64 \][/tex]

2. Calculate [tex]\( 3xy(x + y) \)[/tex]:
[tex]\[ 3xy(x + y) = 3 \cdot 5 \cdot 4 = 60 \][/tex]

3. Subtract [tex]\( 3xy(x + y) \)[/tex] from [tex]\( (x + y)^3 \)[/tex]:
[tex]\[ x^3 + y^3 = 64 - 60 = 4 \][/tex]

Therefore, the value of [tex]\( x^3 + y^3 \)[/tex] is:

[tex]\[ \boxed{4} \][/tex]