[f(x1)+f(x2)]/2=[2^(x1)+2^(x2)]/2
f[(x1+x2)/2]=2^[(x1+x2)/2]=2^(x1/2)*2^(x2/2)
∴[f(x1)+f(x2)]/2-f[(x1+x2)/2]=[2^(x1)+2^(x2)-2*2^(x1)*2^(x2)]/2=[2^(x1/2)-2^(x2/2)]²/2≥0
∴[f(x1)+f(x2)]/2≥f[(x1+x2)/2]
[f(x1)+f(x2)]/2=[2^(x1)+2^(x2)]/2
f[(x1+x2)/2]=2^[(x1+x2)/2]=2^(x1/2)*2^(x2/2)
∴[f(x1)+f(x2)]/2-f[(x1+x2)/2]=[2^(x1)+2^(x2)-2*2^(x1)*2^(x2)]/2=[2^(x1/2)-2^(x2/2)]²/2≥0
∴[f(x1)+f(x2)]/2≥f[(x1+x2)/2]