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# CA Geometry: More trig

66-70, more trigonometry. Created by Sal Khan.

## Want to join the conversation?

• hi sal, for qn 70, why cant we do tan30 = 3(sqrtof3)/x? thx.
• Your way works if you have memorized that tan 30 = 1/sqrt(3), which gives the answer x = 9. Sal always like to start with first principles, so he gave a solution that works out the problem from scratch.
• This might be a stupid question, but I'll ask it anyway. :D

Can you do Soh Cah Toa on more than just triangles; odd-sided polygons(pentagon(5), heptagon(7), nona(9), etc.)?
• Excellent Question!
No, Soh Cah Toa is only used for right triangles. You would have to use a different formula to find the lengths and angles of different shapes.
• Sal, for #70, without drawing out any triangles, how do you find out that C is equal to 2a?
• You can also determine C without memorizing the ratios of the side lengths of a 30-60-90 triangle. They gave us an angle measurement (30) and the length of the opposite side (3sqrt(3)) and C is the hypotenuse. So, using SOH CAH TOA, we get
sin 30 = 3sqrt(3) / C (remember that sin 30 = 1/2)
Solving for C in 1/2 = 3sqrt(3) C gives C = 6sqrt(3)
This follows the ratio of x - 2x - x(sqrt(3))
• step by step solution of complement of 20 degrees 10 minutes and 30 seconds
• There are 60 seconds in a minute.
There are 60 minutes in a degree.
Complements add up to 90 degrees.

I'll use degrees : minutes : seconds for my notation.

Complement of 20 : 10 : 30=

90 : 00 : 00
- 20 : 10 : 30

borrow 60 minutes from a degree, then 60 seconds from 1 minute -

89 : 59 : 60
- 20 : 10 : 30
= 69 : 49 : 30
Answer: 69 degrees, 49 minutes, 30 seconds.

The trick is in the borrowing, just like in regular subtraction, just now you borrow 60, rather than 10.
(1 vote)
• What if we calculate sin of 90 angle. Or better tangent of 90 triangle
(1 vote)
• Im taking the ACT Practice test this saturday and i am wondering if problems like this will be on the real ACT?
• How would you divide 9860 by 22.54 without a calculator when you're bad at math?
• If the angle in question was a right angle, how would sin work?
e.g. sin of (90 degree) = O/H
But because the (O) opposite of the right angle is the same as the (H) hypotenuse, what