# Find the smallest positive

• Mar 26th 2008, 09:30 AM
perash
Find the smallest positive
Find the smallest positive integer solution to
$\displaystyle \tan{19x^{\circ}}=\dfrac{\cos{96^{\circ}}+\sin{96^ {\circ}}}{\cos{96^{\circ}}-\sin{96^{\circ}}}$.
• Mar 26th 2008, 10:16 AM
JaneBennet
$\displaystyle \color{white}.\quad.$ $\displaystyle \frac{\cos{96^\circ}+\sin{96^\circ}}{\cos{96^\circ }-\sin{96^\circ}}$

$\displaystyle =\ \frac{\sin{45^\circ}\cos{96^\circ}+\cos{45^\circ}\ sin{96^\circ}}{\cos{45^\circ}\cos{96^\circ}-\sin{45^\circ}\sin{96^\circ}}$

$\displaystyle =\ \frac{\sin{(96^\circ+45^\circ)}}{\cos{(96^\circ+45 ^\circ)}}$

$\displaystyle =\ \tan{141^\circ}$

$\displaystyle =\ \tan{(141^\circ+180n^\circ)}\quad n\in\mathbb{Z}$

So find the smallest positive integer x such that $\displaystyle 19x\equiv141\!\!\!\!\pmod{180}$. The answer should be $\displaystyle x = 159$. (I worked it out using Excel. (Tongueout))
• Mar 26th 2008, 05:32 PM
ThePerfectHacker
Quote:

Originally Posted by JaneBennet
So find the smallest positive integer x such that $\displaystyle 19x\equiv141\!\!\!\!\pmod{180}$. The answer should be $\displaystyle x = 159$. (I worked it out using Excel. (Tongueout))[/SIZE][/FONT]

Here is a non-excel solution. First $\displaystyle 180 = 2^2\cdot 3^2\cdot 5$.

Thus, this is equivalent to,
$\displaystyle 19x\equiv 141(\bmod 4)$
$\displaystyle 19x\equiv 141(\bmod 9)$
$\displaystyle 19x\equiv 141(\bmod 5)$

Which gets simplified to,
$\displaystyle -x\equiv 1(\bmod 4)$
$\displaystyle x\equiv 6(\bmod 9)$
$\displaystyle -x\equiv 1(\bmod 5)$

The first and third congruences combine since $\displaystyle \gcd(4,5)=1$.

$\displaystyle x\equiv -1(\bmod 20)$
$\displaystyle x\equiv 6(\bmod 9)$

In first congruence subtract 20 once, in second congruence subtract 9 three times. To get*,

$\displaystyle x\equiv - 21(\bmod 20)$
$\displaystyle x\equiv -21(\bmod 9)$

Thus, $\displaystyle x\equiv -21(\bmod 180) \implies x\equiv 159(\bmod 180)$.

*)You can use Chinese Remainder Theorem here, but I do not like to use that theorem, it is too computational. What you see here is my approach to combining modolus.