Answer:
because they made a mistake
1/4
The coefficient is the number attached to a variable.
You need to add 1 3/4, 2 1/3, and 2 3/4.
But before you do that you need to make them decimals.
Add 1.75, 2.33, and 2.75
Equals 6.83
Then subtract it by 20.00
Equals 13.17
Then you make that a decimal
13 17/100
Recall that
sin²(<em>x</em>) = (1 - cos(2<em>x</em>))/2
so that
sin²(7<em>π</em>/8) = (1 - cos(7<em>π</em>/4))/2
sin²(5<em>π</em>/8) = (1 - cos(5<em>π</em>/4))/2
Then
sin²(7<em>π</em>/8) + sin²(5<em>π</em>/8) = (1 - cos(7<em>π</em>/4) + 1 - cos(5<em>π</em>/4))/2
… = 1 - 1/2 (cos(7<em>π</em>/4) + cos(5<em>π</em>/4))
… = 1 - 1/2 (1/√2 - 1/√2)
… = 1
