In an arithmetic sequence:
Tn=t₁+(n-1)d
t₄=t₁+(4-1)d=t₁+3d
t₅=t₁+(5-1)d=t₁+4d
t₆=t₁+(6-1)d=t₁+5d
t₄+t₅+t₆=(t₁+3d) +(t₁+4d)+(t₁+5d)=3t₁+12d
Therefore:
3t₁+12d=300 (1)
t₁₅=t₁+(15-1)d=t₁+14d
t₁₆=t₁+(16-1)d=t₁+15d
t₁₇=t₁+(17-1)d=t₁+16d
t₁₅+t₁₆+t₁₇=(t₁+14d)+(t₁+15d)+(t₁+16d)=3t₁+45d
Therefore:
3t₁+45d=201 (2)
With the equations (1) and (2) we make an system of equations:
3t₁+12d=300
3t₁+45d=201
we can solve this system of equations by reduction method.
3t₁+12d=300
-(3t₁+45d=201)
-----------------------------
-33d=99 ⇒d=99/-33=-3
3t₁+12d=300
3t₁+12(-3)=300
3t₁-36=300
3t₁=300+36
3t₁=336
t₁=336/3
t₁=112
Threfore:
Tn=112+(n-1)(-3)
Tn=112-3n+3
Tn=115-3n
Now, we calculate T₁₈:
T₁₈=115-3(18)=115-54=61
Answer: T₁₈=61
Answer:
B
Step-by-step explanation:
the longest side of a triangle is opposite the largest angle.
∠ Y = 180° - 60° - 43° = 180° - 103° = 77°
then ∠ Y is the largest angle in the triangle , so
side opposite ∠ Y is the longest , that is XZ
3x+5x-12=180
8x=180+12
8x=192
x=192/8
x=24
angle 1 = 5x-12
=5(24)-12
=120-12
=108
angle 2= 3x =3*24=72
angle 1 = angle 9
angle 9= 108
You can count the spirals because they are coloured.
The left handed spirals are in red and you can count 13.
The rifht hanged spirals are in yellow and you can count 21.
Fibonacci sequence is: 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ... From 2 the next number is the sum of the two previous numbers:
2 = 1 + 1
3 = 2 + 1
5 = 3 + 2
8 = 5 + 3
13 = 8 + 5
21 = 13 + 8.
Fibonacci numbers are found in many structures in nature.
We have just found that these cacti enclose Fibonacci numbers 13 and 21.
Answer: 13 and 21
