How do I use variables in the scrapy xpath?

How do I use a variable for scrapy xpath??

For example,

xpath('//*[@id="tb"]/table/tbody/tr[3]/td[1]/text()')

xpath('//*[@id="tb"]/table/tbody/tr[3]/td[2]/text()')

xpath('//*[@id="tb"]/table/tbody/tr[3]/td[3]/text()')

xpath('//*[@id="tb"]/table/tbody/tr[4]/td[1]/text()')

xpath('//*[@id="tb"]/table/tbody/tr[4]/td[2]/text()')

xpath('//*[@id="tb"]/table/tbody/tr[4]/td[3]/text()')

xpath('//*[@id="tb"]/table/tbody/tr[3]/td[7]/text()')

I want to replace tr[3] with a variable. I tried many ways, but I failed.

For example, 'tr["+X+"]'

It's like this.

I don't know even if I look for stack overflow.