Based on the pattern, make a conjecture about the sum of the first 20 positive even numbers.
2 = 2 = 1.2
2+4 = 6 = 2.3
2+4+6 = 12 = 3.4
2+4+6+8 = 20 = 4.5
2+4+6+8+10 = 30 = 5.6
A. the sum is 20.21
B. the sum is 21.22
C. the sum is 19.20
D. the sum is 20.20
2007-09-12 04:33:14 · 3 answers · asked by Anonymous in Science & Mathematics ➔ Mathematics
Notice that the first line has 1*2 (one posit even no.)
the second 2*3 (two posit even nos.)
The third 3*4 (3 posit even nos.)
etc.
So the first twent posit even nos. should be 20*21
2007-09-12 04:45:30 · answer #1 · answered by rrsvvc 4 · 0⤊ 0⤋
it is an AP---
use a simple formula,it may help u in more questions
S(sum)=(n/2)(2a+(n-1)d)
where n=total number=20
d=difference between two no two no.
=4-2=2
S=(20/2)*(2*2+(20-1)*2)
=10*42 or u may write 20*21
2007-09-12 04:52:39 · answer #2 · answered by rahul v 2 · 0⤊ 0⤋
A. 20•21
Sum of 1st n is n(n+1)
2007-09-12 04:38:19 · answer #3 · answered by Philo 7 · 0⤊ 0⤋