79 because the 9 will round up
Yes, you can look at the x-value on the graph and then look at the number of squares up or down the corresponding y-value is
Numbers with 3 divider are squares of primes<span>.</span>
<span>2^2 = 4. Div: 1, 2, 4;</span>
<span>3^3 = 9. Div: 1, 3, 9;</span>
<span>4^4 = 16. Div: 1, 4, 16;</span>
<span>5^5 = 25. Div: 1, 5, 25;</span>
<span>e.t.c.</span>
#sol1
a1=1×4-1=3
a2=2×4-1=7
a3=3×4-1=11
a4=4×4-1=15
a5=5×5-1=19
#sol2
f(1)=1×4-1=3,
f(n+1)=f(n)+4,
so .... you know.