<span>(a + b)2(a - b)=2(a^2-b^2)=2a^2-2b^2</span>
2x^2 + 3y^2 = 14
-x^2 + 2y^2 = 7
2x^2 + 3y^2 = 14
-2x^2 + 4y^2 = 14
2x^2 + 3y^2 = -2x^2 + 4y^2
2x^2 + 2x^2 = -3y^2 + 4y^2
4x^2 = y^2
2x = y
y = 2x
1)a1=1,a50=20,s50=50/2(a1+a50),s50=25.(1+20)=25.21=525
2)a1=1,a100=200,s100=100/2(1+200)=50.201=10050
3)a1=-1,a20=-40,s20=20/2(-1-40)=10.(-41)=-410
4)a1=2,a50=100,s50=50/2(2+100)=25.102=2550