# What is the multiplicative inverse of 6-3i?

ishpiro | Certified Educator

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The multiplicative inverse of an element of a set is another element of this set such that the product of the two elements is a multiplicative identity. In the given set of complex numbers, the multiplicative identity is 1. This is because multiplication by 1 does not change any complex number:

(a + bi)*1 = a + bi for any a and b.

Then, to find multiplicative inverse of 6 - 3i, we need to find the complex number such that the product of 6 - 3i and that number is 1. We can find it by performing division of 1 by 6 - 3i:

1/(6 - 3i) = ?

To find the result, multiply the numerator and denominator of the fraction above by the complex conjugate of 6 - 3i: 6 + 3i.

1/(6 - 3i) = (6+3i)/[(6-3i)(6+3i)]

In the denominator, the product of the two complex conjugates results in a real number:

(6-3i)(6+3i) = 36 - 9i^2 = 36 + 9 = 45. (Keep in mind that i^2 = -1.)

Therefore,

1/(6 - 3i) = (6 + 3i)/45 = 2/15 + (1/15)i.

The multiplicative inverse of 6 - 3i is 2/15 + (1/15)i.

This could be double-checked by performing the multiplication:

(6-3i)(2/15 + (1/15)i) = 12/15 - (6/15)i + (6/15)i - (3/15)i^2 = 4/5+ 1/5 = 1, as expected.

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## Related Questions

justaguide | Certified Educator

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starTop subjects are Math, Science, and Business

The multiplicative inverse of a term x is given by y such that x*y = 1.

Now we have to find the multiplicative inverse of 6 - 3i

Let it be x + yi

(6 - 3i)(x + yi) = 1

=> x + yi  = 1/(6 - 3i)

=> x + yi = (6 + 3i)/(6 - 3i)(6 + 3i)

=> x + yi = (6 + 3i)/ (6^2 + 3^2)

=> x + yi = (6+ 3i)( 36 + 9)

=> x + yi = (6 + 3i)/ 45

=> x + yi = 6/45 + (3/45)i

The multiplicative inverse is 6/45 + (3/45)i

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giorgiana1976 | Student

The multiplicative inverse is the fraction whose numerator is 1 and denominator is the given complex number.

We'll note the inverse as x:

(6-3i)*x = 1

We'll divide by (6-3i) both sides:

x = 1/(6-3i)

Since it is not allowed to keep a complex number to denominator, we'll multiply the ratio by the conjugate of the complex number:

x = (6+3i)/(6-3i)(6+3i)

We'll re-write the denominator as a difference of squares:

(6-3i)(6+3i) = 6^2 - (3i)^2

(6-3i)(6+3i) = 36 - 9i^2

But i^2 = -1:

(6-3i)(6+3i) = 36 + 9

(6-3i)(6+3i) =  45

We'll re-write x:

x = (6+3i)/45

x = 6/45 + 3i/45

We'll simplify and we'll get:

x =  2/15 + i/15

The multiplicative inverse of the complex number 6 - 3i is 2/15 + i/15.

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