90⁰-18⁰ = 72⁰
2⁰+4⁰+6⁰+...+x⁰ = 72⁰ <=> 2(1+2+3+....+k) = 72 <=>
<=> 1+2+3+....+k = 72 : 2 = 36
<=> (k+1)k/2 = 36
k²+k-72 = 0 => k = [-1+√(1+288)] /2 = (-1+17)/2 = 8
Evantaiul are 8 spite.
