Another Hidden Trigonometric Identity

Level pending

Evaluate n = 1 45 ( 1 + tan n π 180 ) \displaystyle\prod_{n=1}^{45}\left(1+\tan{\frac{n\pi}{180}}\right) .


The answer is 8388608.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Tony Lu
Feb 11, 2015

Pair up the products such that their arguments sum to π 4 \frac{\pi}{4} , i.e., ( 1 + tan π 180 ) ( 1 + tan 44 π 180 ) (1+\tan\frac{\pi}{180})(1+\tan\frac{44\pi}{180}) , ( 1 + tan 2 π 180 ) ( 1 + tan 43 π 180 ) (1+\tan\frac{2\pi}{180})(1+\tan\frac{43\pi}{180}) , etc. Let x x and y y be two such arguments. Expanding the product gives ( 1 + tan x ) ( 1 + tan y ) = 1 + tan x + tan y + tan x tan y . (1+\tan x)(1+\tan y)=1+\tan x+\tan y+\tan x\tan y. From tan ( x + y ) = tan x + tan y 1 tan x tan y \tan(x+y)=\frac{\tan x+\tan y}{1-\tan x\tan y} , 1 + tan x + tan y + tan x tan y = 1 + tan ( x + y ) ( 1 tan x tan y ) + tan x tan y . 1+\tan x+\tan y+\tan x\tan y=1+\tan(x+y)(1-\tan x\tan y)+\tan x\tan y. Because x + y = π 4 x+y=\frac{\pi}{4} , ( 1 + tan x ) ( 1 + tan y ) = 1 + 1 tan x tan y + tan x tan y (1+\tan x)(1+\tan y)=1+1-\tan x\tan y+\tan x\tan y = 2. =2. There are 22 such pairs, and 1 + tan 45 π 180 = 2 1+\tan\frac{45\pi}{180}=2 , so the answer is 2 23 \boxed{2^{23}} , or 8388608 \boxed{8388608} .

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...