Question

2-4+6-8+10-12+...+98-100

Answer 1

= 2(1-2+3-4+....+49-50)
= 2(-1×25)
= 2× (-25)
= -50

Answer 2

S1=2+6+10+..98=2(1+3+5+..49)
Iar 1+3+5..+2n-1 = n*n  ->S nr impare ->>> 2n-1=49->n=25
->>S1=2*625= 1250 
S2=-4(1+2+3+...25)= -4  * (25 *26) /2 ->>S2 = -4*325=1300 
Sf=S1+S2=-50