Hello Community,
I'm currently going through "A geometric proof that the derivative of sin x is cos x.". At: http://ocw.mit.edu/courses/mathematics/18-01sc-single-variable-calculus-fall-2010/part-a-definition-and-basic-rules/session-8-limits-of-sine-and-cosine/MIT18_01SCF10_Ses8d.pdf

The one sentence I don't understand is:
"But |PR| is just the vertical distance between
tween Q and P".

Shouldn't QRP be a 90-45-45 triangle. Making RP=QP impossible?

Question

Hello Community,
I'm currently going through "A geometric proof that the derivative of sin x is cos x.". At: http://ocw.mit.edu/courses/mathematics/18-01sc-single-variable-calculus-fall-2010/part-a-definition-and-basic-rules/session-8-limits-of-sine-and-cosine/MIT18_01SCF10_Ses8d.pdf

The one sentence I don't understand is:
"But |PR| is just the vertical distance between 
tween Q and P".

Shouldn't QRP be a 90-45-45 triangle. Making RP=QP impossible?

anonymous · Answer

He is not saying that RP=QP, he is saying that the distance on the y-axis (the vertical distance) is the same (d in the drawing). |dw:1346054602482:dw|