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Evaluate \((2-5i)(p+q)(i)\) when \(p=2\) and \(q=5i\).

A. \(29i\)
B. \(29i - 20\)
C. [tex]\(-21i\)[/tex]

Sagot :

To solve the expression \((2 - 5i)(p + q)(i)\) given that \( p = 2 \) and \( q = 5i \), follow these steps:

1. Substitute the values for \( p \) and \( q \):

Given:
[tex]\[ p = 2 \][/tex]
[tex]\[ q = 5i \][/tex]

2. Form the second term:

Substitute \( p \) and \( q \) into \( p + q \):
[tex]\[ p + q = 2 + 5i \][/tex]

3. Multiply the first term by the second term:

The expression becomes:
[tex]\[ (2 - 5i)(2 + 5i) \][/tex]

To expand the product, use the distributive property (FOIL method):
[tex]\[ (2 - 5i)(2 + 5i) = 2 \cdot 2 + 2 \cdot 5i - 5i \cdot 2 - 5i \cdot 5i \][/tex]
[tex]\[ = 4 + 10i - 10i - 25i^2 \][/tex]

Note that \( i^2 = -1 \):
[tex]\[ = 4 + 10i - 10i - 25(-1) \][/tex]
[tex]\[ = 4 + 25 \][/tex]
[tex]\[ = 29 \][/tex]

4. Multiply the result by \( i \):

Now, take the result \( 29 \) and multiply it by \( i \):
[tex]\[ 29 \cdot i = 29i \][/tex]

Therefore, the evaluated expression \((2 - 5i)(2 + 5i)(i)\) is:

[tex]\[ \boxed{29i} \][/tex]

This matches the true result described in the problem.