12x1= 12
12+ 11= 23
23/12 + 9/12 = 32/12 or 3 8/12
You have to use the method of FOIL (first, outer, inner and Last)
9b*9b is first
9b*1 is outer
-1*9b is inner
-1*1 is last
Together, they form the expression 81b^2 + 9b - 9b - 1
Collect like terms and the answer is 81b^2 - 1
Since this is an arithmetic sequence, you can conclude that
the addition to the number is constant.
Let x = the number asked
k = constant
number added to the sequence
7 + k = x
x + k = 17
Solve the system of equation:
7 + k + k = 17
7 + 2k = 17
2k = 10
k = 5
Therefore, the missing number is 12 (7 + 5).
Sin 45=x/s
s=x/sin 45
s=x√2
sin 60=s/12√6
(√3/2)(12√6)=x√2
(18√2)/√2=x
x=18