Step by step
The question
Question: find the area of the triangle with sides 11 and 8, and the angle 40° between them.
1 Find the height
The area needs a height.
Draw it from the top corner straight down to the base, at a square corner: h.
The small right triangle on the left has the side 8 as its longest side, and h across from the 40° angle.
SOH: sin is the opposite over the hypotenuse.
sin 40° = h/8.
To remove the 8 at the bottom, we multiply by 8.
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.
8/8 is 1.
So just h is left.
So h = 8 × sin 40°.
2 Write the rule
The area of a triangle is a half times the base times the height.
The base is 11, the height is 8 × sin 40°.
So the rule is ½ × a × b × sin C: two sides and the sine of the angle between them.
Careful!
The angle must be the one between the two sides we use.
Another angle gives a wrong height.
3 Put the numbers in
Now put the numbers in: the two sides 11 and 8, and sin 40°.
4 Work it out
11 times 8 is 88.
Half of 88 is 44.
The calculator gives sin 40°: about 0.6428.
Put the number in.
That is about 28.283.
To the nearest tenth, the area is about 28.3 square units.
5 Check
Let's check another way: use the side 8 as the base.
Its height is 11 times sin 40°: about 7.07.
A half times 8 times that is about 28.3.
The same area!
So it is right.
Let's look back.
First: find the height.
Then: write the rule.
Then: put the numbers in.
Then: work it out.
Last: check.