# Math Help - Triangle (Proving)

1. ## Triangle (Proving)

I'm getting very confused...there are too many formula for this!

Prove:
1. sin (A+B) + cos (A-B) $\equiv$ (sin A+ cos A)(sin B + cos B)

2. tan (A+B) - tan A $\equiv \frac{sin B}{cos A cos (A+B)}$

And one more extra:
3. In triangle ABC, AB=p, BC=3p, and $\angle ABC=60^o$. The bisector of the angle ABC meets AC at the point T. Find AT and CT in terms of p.

Answer: $AT=\frac{p}{4}\sqrt{7}; CT=\frac{3p}{4}\sqrt{7}$

2. Originally Posted by cloud5
I'm getting very confused...there are too many formula for this!

Prove:
1. sin (A+B) + cos (A-B) $\equiv$ (sin A+ cos A)(sin B + cos B)

2. tan (A+B) - tan A $\equiv \frac{sin B}{cos A cos (A+B)}$

And one more extra:
3. In triangle ABC, AB=p, BC=3p, and $\angle ABC=60^o$. The bisector of the angle ABC meets AC at the point T. Find AT and CT in terms of p.

Answer: $AT=\frac{p}{4}\sqrt{7}; CT=\frac{3p}{4}\sqrt{7}$
HI

(1) $\sin A\cos B+\cos A\sin B+\cos A\cos B+\sin A\sin B$

$\sin A(\cos B+\sin B)+\cos A(\cos B+\sin B)$

$(\sin A+\cos A)(\sin B+\cos B)$

(2) $\frac{\sin (A+B)}{\cos (A+B)}-\frac{\sin A}{\cos A}$

$\frac{\sin (A+B)\cos A-\sin A\cos (A+B)}{\cos A\cos (A+B)}$

$\frac{\sin A\cos B\cos A+\cos A\sin B\cos A-\cos A\cos B\sin A+\sin^2 A\sin B}{\cos A\cos (A+B)}$

$\frac{(\cos^2 A+\sin^2 A)\sin B}{\cos A\cos (A+B)}$

(3) Make a sketch first . The ratio of AT:TC=1:3

By the cosine rule , $AC^2=p^2+(3p)^2-2p(3p)\cos 60$

$AC=p\sqrt{7}$

$AT=\frac{1}{4}AC=\frac{1}{4}p\sqrt{7}$

sure you can find TC now .

$\frac{\sin A\cos B\cos A+\cos A\sin B\cos A-\cos A\cos B\sin A+\sin^2 A\sin B}{\cos A\cos (A+B)}$

$\frac{(\cos^2 A+\sin^2 A)\sin B}{\cos A\cos (A+B)}$

Can you explain what happen between those steps?

4. Originally Posted by cloud5
Can you explain what happen between those steps?
sure , notice that \cos A\cos B\sin A cancels off and you are left with

$\frac{\cos^2 A\sin B+\sin^2 A\sin B}{\cos A\cos (A+B)}$

then bring out the common factor , sin B .

$AC=p\sqrt{7}$

$AT=\frac{1}{4}AC=\frac{1}{4}p\sqrt{7}$

sure you can find TC now .
I don't understand this part... how you got $\sqrt{7}$?

6. Originally Posted by cloud5
I don't understand this part... how you got $\sqrt{7}$?
Note that cos 60=1/2

$AC^2=10p^2-6p^2(\frac{1}{2})$

$
AC^2=7p^2 so AC=p\sqrt{7}
$

$AT=\frac{1}{4}AC=\frac{1}{4}p\sqrt{7}$
One more question, how you get $\frac{1}{4}$?

8. Originally Posted by cloud5
One more question, how you get $\frac{1}{4}$?
because AT:TC is in the ratio of 1:3 , the 'total' for AC is 4 .

so AT = 1/4 AC