Answer:
a.
f(1)=6; f(n)=6+d(n-1), n>0
Explanation:
We are given that
First layer has squares, a=6
Second layer has squares, a2=12
We have to find an arithmetic explicit formula to determine the number of squares in each layer.
![d=a_2-a_1=12-6](https://img.qammunity.org/2022/formulas/mathematics/high-school/6j82yz4ulxhkg90ma3unk6gkqkw2z70to2.png)
nth term of an A.P
![a_n=a+(n-1)d](https://img.qammunity.org/2022/formulas/mathematics/high-school/z3ggkqmhdr30c6f6y2dgecyqzzjyb67v0v.png)
Substitute the value of a
Now, we get
![a_n=6+(n-1)d](https://img.qammunity.org/2022/formulas/mathematics/high-school/pygdk9w09upn67q79z3vzrzr9wy7p759vh.png)
f(1)=a=6
![a_n=f(n)=6+d(n-1)](https://img.qammunity.org/2022/formulas/mathematics/high-school/ubfpo1lp4cajfc8uqmz8918p3ushx6wq82.png)
Hence, option a is correct.
a.
f(1)=6; f(n)=6+d(n-1), n>0