Question 1150756
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Expressing the third and ninth terms in terms of the first term and common difference, as both the other tutors did, adds unnecessary steps (and time) to finding the solution.<br>
The ninth term is the third term, plus 9-3=6 times the common difference:<br>
{{{10x-8y = (4x-2y)+6d}}}
{{{(10x-8y)-(4x-2y) = 6d}}}
{{{6x-6y = 6d}}}
{{{d = x-y}}}<br>