Question

A Christmas tree is decorated with bulbs such that first branch has 5 bulbs, 7 in the second, 9 in the third and so on. If the tree is to have a total of 525 bulbs on it, how many branches must be used in his design?

Answer 1

Answer:

21 branches

Step-by-step explanation:

Given : d = 7-5= 2 , a= 5

Sn = 525

Substitute the above in the formula

Sn = n/2 (2a + (n-1)d)

525 = n/2 (2*5 + (n-1) 2)

525 = n/2 (10+2n-2)

525= n/2 (8+2n)

525*2=8n+2n²

1050= 8n +2n²

n²+4n-525

n²+25n -21n -525

n(n+25) -21(n+25)

(n+25)(n-21)

n= -25,21

since the number of branches cannot be in negative, we take n= 21