If the initial arithmetic progrssion is a +i·d, then the condition is(a + 15d)² / (a + 13d)² = (a + 13d)² / (a + 12d)²which gives you(a + 15d)² · (a + 11d)² = (a + 13d)² · (a + 13d)².There are two ways to get rid of the square. Try both.
(a + 15d)² / (a + 13d)² = (a + 13d)² / (a + 12d)²
which gives you
(a + 15d)² · (a + 11d)² = (a + 13d)² · (a + 13d)².
There are two ways to get rid of the square. Try both.