Multiple choice The sum of $7$ terms of the series $1^2-2^2+3^2-4^2+5^2-6^2+...$ is $-21$ $15$ $28$ $-35$ Reveal answer Fill a bubble to check yourself C Correct answer Explanation The series is (1^2-2^2) + (3^2-4^2) + (5^2-6^2) + 7^2. This is (-3) + (-7) + (-11) + 49 = -21 + 49 = 28.