Let's use the formula for the
nth term of .
an=a1+(n−1)d
Since we know the
d=-5 and
a12=22, we can substitute
-5 for
d, 12 for
n, and
22 for
an.
22=a1+(12−1)(-5)
Since
a1 is the only unknown value left, we can can calculate its value.
22=a1+(12−1)(-5)
22=a1+(11)(-5)
22=a1+(-55)
77=a1
a1=77
The first term equals
77.