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Simplify the following expression into the form [tex]a + b i[/tex], where [tex]a[/tex] and [tex]b[/tex] are rational numbers.

[tex](3 + 7i)(-2 - 2i) - 4i(5 - 9i)[/tex]

Sagot :

To simplify the given expression [tex]\((3+7i)(-2-2i) - 4i(5-9i)\)[/tex] into the form [tex]\(a+bi\)[/tex], where [tex]\(a\)[/tex] and [tex]\(b\)[/tex] are rational numbers, let's break it down step-by-step.

### Simplifying [tex]\((3 + 7i)(-2 - 2i)\)[/tex]:
First, we distribute each term in the first expression:

[tex]\[ (3 + 7i)(-2 - 2i) = 3(-2) + 3(-2i) + 7i(-2) + 7i(-2i) \][/tex]

Calculating each part separately:

[tex]\[ 3(-2) = -6 \][/tex]
[tex]\[ 3(-2i) = -6i \][/tex]
[tex]\[ 7i(-2) = -14i \][/tex]
[tex]\[ 7i(-2i) = -14i^{2} \][/tex]

Since [tex]\(i^2 = -1\)[/tex], we have:

[tex]\[ 7i(-2i) = -14(-1) = 14 \][/tex]

Combining these results:

[tex]\[ (3 + 7i)(-2 - 2i) = -6 - 6i - 14i + 14 \][/tex]

Grouping the real and imaginary parts together:

[tex]\[ -6 + 14 + (-6i - 14i) = 8 - 20i \][/tex]

### Simplifying [tex]\(-4i(5 - 9i)\)[/tex]:
Next, we distribute each term in the second expression:

[tex]\[ -4i(5 - 9i) = -4i(5) + (-4i)(-9i) \][/tex]

Calculating each part separately:

[tex]\[ -4i(5) = -20i \][/tex]
[tex]\[ -4i(-9i) = 36i^{2} \][/tex]

Again, using [tex]\(i^2 = -1\)[/tex]:

[tex]\[ 36i^{2} = 36(-1) = -36 \][/tex]

So, combining these results:

[tex]\[ -4i(5 - 9i) = -20i - 36 \][/tex]

### Combining both parts:
Now we subtract the second simplified expression from the first one:

[tex]\[ (8 - 20i) - (-20i - 36) \][/tex]

Rewriting this, we get:

[tex]\[ 8 - 20i + 20i + 36 \][/tex]

Simplifying further:

[tex]\[ 8 + 36 = 44 \quad \text{(Real part)} \][/tex]
[tex]\[ -20i + 20i = 0i \quad \text{(Imaginary part)} \][/tex]

Thus, the simplified expression is:

[tex]\[ \boxed{-28 + 0i} \][/tex]