ElectricPavlov

I'd go with Guest #1 answer.    Omi67:   20 40 60 are not consecutive even integers !  

Depends on how much you paid for the first sinus.....generally a discount is given for the cosinus.

....and buy a lawnmower.

According to your equation:   2y = -5   so    y = -2.5

ALSO:     x + 2x - 3y = -5

               3x - 3(-2.5) = -5

               3x + 7.5      = -5

                3 x = -12.5

                   x = - 125/30 =  - 4 1/6

IF you entered your Q correctly !

Nice !

Still disagree.  At the beginning of the 7th year 'd' students have been added.     At the beginning of the 2nd year 'd' students have been added.       We'll just have to agree, as the English language is full of 'nuances' and unclear meaning, to disagree.  G'Day!    Thanx  !

I'll have to disagree with Guest #2.   

Your equations do not work if d=43  and a = 234

a(n) =a(1) + (n-1)d      results in a(7) = 492                 the correct answer needs to be 449

Your mistake is here:   449 =a(1) + (6-1)43      where '6' should be '7'      AND at year 7 the students are at 449  SIX (not SEVEN as you described)more terms later , at year 13, the total is 750.  

Soooooo the 'common difference' as you describe it is  (750-449)/6 = 50 1/6  (NOT 43).

Honest mistakes we all make (frequently!)    

x = amount at 8%

30,000 - x  = amount at 5%

x(.08) - 840 = (30,000 - x)(.05)     Solve for 'x' and '30,000 - x'

a = #students first year

a + d = #students SECOND year

for all of the following 'n' years

#students = a + (n-1) d

For 7   years  #students = 449 = a + (6)d            (1)

For 13 years  #students = 750 = a + (12)d          (2)

From (1)     a = 449 - 6d 

Substitute into (2):      750 = (449 - 6d) + 12d  and solve for 'd'

                                    750-449 = -6d + 12d

                                     301 = 6 d  

                                       d=50 1/6  

     (not sure how you add 50 1/6 students per year...must be an AVERAGE per year)

Substitute this value of 'd' into one of the equations to find 'a' = ?

(1)    449 = a + 6 (50 1/6)

         449 = a + 301

         449 - 301 = a             a = 148  

Circumference is  pi x DIAMETER.

 You are given RADIUS = .9 cm   DIAMETER is 2 x RADIUS = 2 x .9 = 1.8 cm

pi x DIAMETER = 3.14 x 1.8cm = 5.7 cm  (to ONE decimal place)