Matrix Transformations, Rotate 90 Degrees

A figure on the coordinate plane (pre-image) is represented by a matrix. The rotated figure (image) is determined and represented by a second matrix. An equation is setup to allow the student to figure out the "90 degree rotation" transformation matrix.

[methical__]

not gonna lie, for a brief moment I thought about moving the paper a bit to the left with my cursor.

[____________________________.x]

Ah, this is so much clearer than the rest

[mamdouhmorad]

wish you were my math teacher back in school :D

[AlexanderJonesYoutube]

How would I use this to rotate it by 45 for example?

[TheAmeer]

Hey man I liked your video, but you need better recording audio. The noise was really not nice. Anyway, I subbed and I hope you find that and fix it. Hoping I made the right choice

[FutureAIDev]

Say I have a point P that I want to rotate about a center C, which is distance D away, on a sphere with radius R by the amount A. The rotation is clockwise. How do I do this? The points are described with latitude and longitude.

[giancarloandrebravoabanto]

very good explanation. what math topic is this? to search for it

[arcan]

Isn't this counter-clockwise rotation?

[quintijnvanheek]

OOOOHHH MA GODDDD you're a fuckin saviour thank you so much!!

[RameshNatarajan]

Hi, If the generic rotation matrix is
[cos(theta), -sin(theta)]
[sin(theta), cos(theta)]
and we plugin 90 for theta, we don't get
[0, -1]
[1, 0]
since sin(90) is 0.8939967 and cos(90) is -0.4480736. 
Can someone pl help me with this? Thanks in advance.