# Thread: Trig problem help plzzz

1. ## Trig problem help plzzz

Hi so i have this trig problem for homework and I can't get it, can you please help?
Here it is:
If A is an obtuse angle in a triangle and sinA= 5/13, calculate the exact balue of sin2A.
Could you please do it step by step.
Thank you!!

2. Originally Posted by kaleyy =]
Hi so i have this trig problem for homework and I can't get it, can you please help?
Here it is:
If A is an obtuse angle in a triangle and sinA= 5/13, calculate the exact balue of sin2A.
Could you please do it step by step.
Thank you!!
You can construct a triangle with an angle A and opposite side 5 with hypotenuse 13.

You can then use Pythagoras Theorem to find the missing side which turns out to be 12.

We can then say cos(A) = adj/hyp = 12/13

$\displaystyle sin(2A) = 2sinAcosA = 2 \times \frac{5}{13} \times \frac{12}{13} = \frac{120}{169}$

I'm not too sure how the quadrants come into it so you may want to wait for someone more experienced with that part