The first item of an arithmetic sequence is 6, and the eighth item is 55. What is the tolerance?
一个等差数列的首项是6，第8项是55，公差是多少．
<gadget id="calculator">55 - 6</gadget>
<output>49</output>

<gadget id="calculator">8 - 1</gadget>
<output>7</output>

<gadget id="calculator">49 / 7</gadget>
<output>7</output>

<result>7</result>