Question

find the sum of 31+27+23+....+(-45)​

Answer 1

n=(tn-ti)/d + I

(-45-31)/-4. +1=20 (No. of terms)

Sum=n/2{2a+(n-1) -4}

19/2{62+(20-1)-4}

19/2{(62+19)-4}= 19/2x (81x-4)

= -3078

hope it helps