Question

how many terms of the ap:-6,-11/2,-5 are needed to give a sum of -25? Explain the double answer

Answer 1

sum = n/2( a + l) 

- 25 = n/2 ( -12+ (n-1) (1/2) ) 

n² - 25 n + 100 = 0

n = 20 or n = 5

Answer 2

after putting the formula of Sum of AP you will get an quadratic equation "n*n -25n+100=0"