Answer:
![\tan(a-b)=\frac{2\sqrt{5}-20\sqrt{3}}{5+8\sqrt{15}}](https://tex.z-dn.net/?f=%5Ctan%28a-b%29%3D%5Cfrac%7B2%5Csqrt%7B5%7D-20%5Csqrt%7B3%7D%7D%7B5%2B8%5Csqrt%7B15%7D%7D)
Step-by-step explanation:
I'm going to use the following identity to help with the difference inside the tangent function there:
![\tan(a-b)=\frac{\tan(a)-\tan(b)}{1+\tan(a)\tan(b)}](https://tex.z-dn.net/?f=%5Ctan%28a-b%29%3D%5Cfrac%7B%5Ctan%28a%29-%5Ctan%28b%29%7D%7B1%2B%5Ctan%28a%29%5Ctan%28b%29%7D)
Let
.
With some restriction on
this means:
![\sin(a)=\frac{2}{3}](https://tex.z-dn.net/?f=%5Csin%28a%29%3D%5Cfrac%7B2%7D%7B3%7D)
We need to find
.
is a Pythagorean Identity I will use to find the cosine value and then I will use that the tangent function is the ratio of sine to cosine.
![(\frac{2}{3})^2+\cos^2(a)=1](https://tex.z-dn.net/?f=%28%5Cfrac%7B2%7D%7B3%7D%29%5E2%2B%5Ccos%5E2%28a%29%3D1)
![\frac{4}{9}+\cos^2(a)=1](https://tex.z-dn.net/?f=%5Cfrac%7B4%7D%7B9%7D%2B%5Ccos%5E2%28a%29%3D1)
Subtract 4/9 on both sides:
![\cos^2(a)=\frac{5}{9}](https://tex.z-dn.net/?f=%5Ccos%5E2%28a%29%3D%5Cfrac%7B5%7D%7B9%7D)
Take the square root of both sides:
![\cos(a)=\pm \sqrt{\frac{5}{9}}](https://tex.z-dn.net/?f=%5Ccos%28a%29%3D%5Cpm%20%5Csqrt%7B%5Cfrac%7B5%7D%7B9%7D%7D)
![\cos(a)=\pm \frac{\sqrt{5}}{3}](https://tex.z-dn.net/?f=%5Ccos%28a%29%3D%5Cpm%20%5Cfrac%7B%5Csqrt%7B5%7D%7D%7B3%7D)
The cosine value is positive because
is a number between
and
because that is the restriction on sine inverse.
So we have
.
This means that
.
Multiplying numerator and denominator by 3 gives us:
![\tan(a)=\frac{2}{\sqrt{5}}](https://tex.z-dn.net/?f=%5Ctan%28a%29%3D%5Cfrac%7B2%7D%7B%5Csqrt%7B5%7D%7D)
Rationalizing the denominator by multiplying top and bottom by square root of 5 gives us:
![\tan(a)=\frac{2\sqrt{5}}{5}](https://tex.z-dn.net/?f=%5Ctan%28a%29%3D%5Cfrac%7B2%5Csqrt%7B5%7D%7D%7B5%7D)
Let's continue on to letting
.
Let's go ahead and say what the restrictions on
are.
is a number in between 0 and
.
So anyways
implies
.
Let's use the Pythagorean Identity again I mentioned from before to find the sine value of
.
![\cos^2(b)+\sin^2(b)=1](https://tex.z-dn.net/?f=%5Ccos%5E2%28b%29%2B%5Csin%5E2%28b%29%3D1)
![(\frac{1}{7})^2+\sin^2(b)=1](https://tex.z-dn.net/?f=%28%5Cfrac%7B1%7D%7B7%7D%29%5E2%2B%5Csin%5E2%28b%29%3D1)
![\frac{1}{49}+\sin^2(b)=1](https://tex.z-dn.net/?f=%5Cfrac%7B1%7D%7B49%7D%2B%5Csin%5E2%28b%29%3D1)
Subtract 1/49 on both sides:
![\sin^2(b)=\frac{48}{49}](https://tex.z-dn.net/?f=%5Csin%5E2%28b%29%3D%5Cfrac%7B48%7D%7B49%7D)
Take the square root of both sides:
![\sin(b)=\pm \sqrt{\frac{48}{49}](https://tex.z-dn.net/?f=%5Csin%28b%29%3D%5Cpm%20%5Csqrt%7B%5Cfrac%7B48%7D%7B49%7D)
![\sin(b)=\pm \frac{\sqrt{48}}{7}](https://tex.z-dn.net/?f=%5Csin%28b%29%3D%5Cpm%20%5Cfrac%7B%5Csqrt%7B48%7D%7D%7B7%7D)
![\sin(b)=\pm \frac{\sqrt{16}\sqrt{3}}{7}](https://tex.z-dn.net/?f=%5Csin%28b%29%3D%5Cpm%20%5Cfrac%7B%5Csqrt%7B16%7D%5Csqrt%7B3%7D%7D%7B7%7D)
![\sin(b)=\pm \frac{4\sqrt{3}}{7}](https://tex.z-dn.net/?f=%5Csin%28b%29%3D%5Cpm%20%5Cfrac%7B4%5Csqrt%7B3%7D%7D%7B7%7D)
So since
is a number between
and
, then sine of this value is positive.
This implies:
![\sin(b)=\frac{4\sqrt{3}}{7}](https://tex.z-dn.net/?f=%5Csin%28b%29%3D%5Cfrac%7B4%5Csqrt%7B3%7D%7D%7B7%7D)
So
.
Multiplying both top and bottom by 7 gives:
.
Let's put everything back into the first mentioned identity.
![\tan(a-b)=\frac{\tan(a)-\tan(b)}{1+\tan(a)\tan(b)}](https://tex.z-dn.net/?f=%5Ctan%28a-b%29%3D%5Cfrac%7B%5Ctan%28a%29-%5Ctan%28b%29%7D%7B1%2B%5Ctan%28a%29%5Ctan%28b%29%7D)
![\tan(a-b)=\frac{\frac{2\sqrt{5}}{5}-4\sqrt{3}}{1+\frac{2\sqrt{5}}{5}\cdot 4\sqrt{3}}](https://tex.z-dn.net/?f=%5Ctan%28a-b%29%3D%5Cfrac%7B%5Cfrac%7B2%5Csqrt%7B5%7D%7D%7B5%7D-4%5Csqrt%7B3%7D%7D%7B1%2B%5Cfrac%7B2%5Csqrt%7B5%7D%7D%7B5%7D%5Ccdot%204%5Csqrt%7B3%7D%7D)
Let's clear the mini-fractions by multiply top and bottom by the least common multiple of the denominators of these mini-fractions. That is, we are multiplying top and bottom by 5:
![\tan(a-b)=\frac{2 \sqrt{5}-20\sqrt{3}}{5+2\sqrt{5}\cdot 4\sqrt{3}}](https://tex.z-dn.net/?f=%5Ctan%28a-b%29%3D%5Cfrac%7B2%20%5Csqrt%7B5%7D-20%5Csqrt%7B3%7D%7D%7B5%2B2%5Csqrt%7B5%7D%5Ccdot%204%5Csqrt%7B3%7D%7D)
![\tan(a-b)=\frac{2\sqrt{5}-20\sqrt{3}}{5+8\sqrt{15}}](https://tex.z-dn.net/?f=%5Ctan%28a-b%29%3D%5Cfrac%7B2%5Csqrt%7B5%7D-20%5Csqrt%7B3%7D%7D%7B5%2B8%5Csqrt%7B15%7D%7D)