We have, an = (−1)n−15n+1
Substituting n = 1, 2, 3, 4, 5, we get
a1 = (−1)1−151+1 = (−1)052 = 25 , a2 = (−1)2−152+1 = (−1)153 = - 125
a3 = (−)3−153+1 = (−1)254 = 625, a4 = (−1)4−15+1 = (−1)353 = - 3125
a5 = (−1)5−155+1 = (−1)456 = 15625
∴ The first five terms are 25, – 125, 625, –3125, 15625.