Question

find the some of 31 + 27 + 23 +................+(-45)​

Answer 1

Answer:

-140

Step-by-step explanation:

Use AP.$a_{n}  = a + (n-1)*d \\-45  = 31 + (n-1)*(-4)  \\(n-1)*(4)  = 31+45\\\\n-1  = \frac{31+45}{4}\\\\n-1 = 76/4 \\n - 1 = 19\\n = 20\\\\sum = (n/2)(2a + (n-1)d)\\\\sum = (20/2)(2*31 +(20-1)* -4)sum = -140$

Answer 2

Answer:

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