Answer:
7A−(I + A)³
=7A−(1³ + A³ +3.I.A² +3.1².A)
=7A−(I+ A².A+3A² +34)
= 7A-(I+A.A+3A+34) (*: A² = A)
=7A-(I+ A² +6A)
= 7A-(I+A+64)
=7A-(1+7A)
=7A-I-7A
=-1
You are correct there's at least one obtuse angle
The acceleration of the bookshelf is equal to 1.30 
<u>Given the following data:</u>
- Mass of bookshelf = 51.5 kilogram
To determine the acceleration of the bookshelf, we would apply Newton's Second Law of Motion:
Newton's Second Law of Motion states that the acceleration of an object is directly proportional to the net force acting on the object while being inversely proportional to its mass.
Mathematically, Newton's Second Law of Motion is given by this formula;

Substituting the given parameters into the formula, we have;

Acceleration = 1.30 
Read more on net force here: brainly.com/question/1121817
Using the law of cosins
sin(angle) = Opposite Leg / Hypotenuse
The angle is given as 35 degree and the hypotenuse is given as 30 feet.
Sin(35) = X ( opposite leg) / 30
X = 30 * sin(35)
x = 17.2 feet.
Total height = 17.2 + 6.5 = 23.7, round to 24 feet.
The answer is 24 feet.