Step by step
The question
Question: find sin A, when cos A = 20/29.
1 Draw the triangle
sin, cos and tan belong to an angle of a right triangle.
So we draw a right triangle with the angle A.
2 Use what cos A means
The opposite side is across from A.
We write opp, adj and hyp for short.
The adjacent side is next to A.
It is not the hypotenuse.
The hypotenuse is the longest side.
It is across from the right angle.
cos A is the adjacent side over the hypotenuse.
The question says cos A = 20/29.
So adj over hyp is 20/29.
Every right triangle with the angle A has the same shape.
A bigger one could have 40 and 58.
40/58 is 20/29 too.
So we can use the small one: adj = 20 and hyp = 29.
3 Find the missing side
The opposite side is missing.
We call it x.
Pythagoras: in a right triangle, the two short sides squared add up to the hypotenuse squared.
Now put the numbers in: opp = x, adj = 20 and hyp = 29.
20² means 20 times 20.
That is 400.
29² means 29 times 29.
That is 841.
Write the squares: x² + 400 = 841.
We want x² by itself.
The 400 is in the way.
To remove 400, we take away 400.
We do it on both sides.
Why both sides?
The = sign means both sides are the same amount.
If we change only one side, they are not the same any more.
400 and − 400 make zero.
Cross them out.
841 minus 400 is 441.
x² is 441.
So x times x is 441.
That means x is the square root of 441.
The square root of 441 is the plus number that times itself makes 441.
21 times 21 is 441.
−21 times −21 is 441 too, but the √ sign always means the plus one.
So x = 21.
Careful!
We cannot just take away the sides.
Pythagoras works with the squares: 841 − 400 = 441.
Then the square root.
4 Write sin A
sin A is the opposite side over the hypotenuse.
Put in opp = 21 and hyp = 29.
So sin A = 21/29.
5 Check
Let's check with the calculator.
Set the calculator to degrees first.
The cos⁻¹ button turns cos A back into the angle: A is about 46.4°.
sin 46.4° is about 0.72.
And 21/29 is about 0.72.
The same.
So sin A = 21/29 is right!
Let's look back.
First: draw the triangle.
Then: use what cos A means.
Then: find the missing side.
Then: write sin A.
Last: check.