Proving (cosx+isinx)^2: A Simple Complex Number Problem | Homework Solution

In summary, the conversation discusses how to show that (cosx+isinx)^2 is equal to cos2x + isin2x by using the basic double angle formulas for cos2x and sin2x. The conversation also touches on how to simplify the expression and how to incorporate the formula for sin2x=2sinxcosx.
  • #1
nirvana1990
46
0

Homework Statement


Show that: (cosx+isinx)^2= cos2x + isin2x


Homework Equations


i^2=-1


The Attempt at a Solution



Well, here's my attempt!
(cosx+isinx)^2=(cosx+isinx)(cosx+isinx)
=(cos^2x)+(2[isinxcosx])+(i^2sin^2x)
=(cos^2x)+(2[isinxcosx])-sin^2x

p.s. when i wrote cos^2x, for example, I meant cos squared, multiplied by x.
 
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  • #2
Have a look at the basic double angle formulas for cos2x and sin2x, and all will be revealed!
 
  • #3
Ooh thanks that was quite helpful but now i seem to have over-simplified somehow!
I got: cos^2x-sin^2x+2isin2xcos2x
=cos2x+2isin2xcos2x (by using cos^2x-sin^2x=cos2x)
Then would you divide by cos2x to give: 1+2isin2x? Or should I use sin2x=2sinxcosx somewhere??
 
  • #4
nirvana1990 said:
Ooh thanks that was quite helpful but now i seem to have over-simplified somehow!
I got: cos^2x-sin^2x+2isin2xcos2x
No, you did not have that before- you are getting ahead of yourself!
You had cos2x- sin2x+ i(2 sin x cos x), NOT "2cos 2x sin 2x.

=cos2x+2isin2xcos2x (by using cos^2x-sin^2x=cos2x)
Then would you divide by cos2x to give: 1+2isin2x? Or should I use sin2x=2sinxcosx somewhere??
Since you do have 2 sin x cos x, it should be obvious exactly where to use that!
 
  • #5
Yes thanks I realized my error this morning after doing many numerical examples!
cos^2x-sin^2x+ 2(isinxcosx)= cos2x+i(2sinxcosx)=cos2x+isin2x

thanks for the help!
 

FAQ: Proving (cosx+isinx)^2: A Simple Complex Number Problem | Homework Solution

What is a complex number?

A complex number is a number that has both a real and an imaginary component. It is typically expressed in the form a + bi, where a is the real part and bi is the imaginary part, represented by the imaginary unit i (where i = √-1).

What is the difference between a real and an imaginary number?

A real number is any number that can be plotted on a number line, including positive and negative numbers, fractions, and decimals. An imaginary number, on the other hand, is any number that involves the square root of a negative number, such as √-4. It cannot be plotted on a number line and is typically represented by the letter i.

How do you perform operations on complex numbers?

To add or subtract two complex numbers, simply combine the real parts and the imaginary parts separately. For multiplication, use the FOIL method as you would for binomials. To divide two complex numbers, multiply the numerator and denominator by the complex conjugate of the denominator, which is the same number with the sign of the imaginary part flipped.

Can complex numbers be graphed on a coordinate plane?

Yes, complex numbers can be graphed on a coordinate plane, with the real part representing the x-axis and the imaginary part representing the y-axis. The point where the two axes intersect represents the origin, or 0 + 0i.

What are some real-life applications of complex numbers?

Complex numbers are used in a variety of fields, including engineering, physics, and signal processing. They are also used in the study of electric circuits, fluid dynamics, and quantum mechanics. In everyday life, complex numbers can be used to describe the motion of objects, analyze financial data, and even create realistic computer-generated images in movies and video games.

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