Question

1+2+3+..........+20=_______?

Answer 1

in total there are 20 terms

sum up to n terms in an AP =n/2(a+l), where n is the number of terms and a is the first term of AP and l is the last term of the AP
therefore sum of first 20 terms =20/2(1+20)
=10(21)
=210
therefore 1+2+3+......+20=210



I hope this will help you