Question 942470
Find the normal of each plane, then use the dot product.
{{{N[1]}}}=({{{3}}},{{{-1}}},{{{1}}})
{{{N[2]}}}=({{{1}}},{{{2}}},{{{2}}})
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{{{N[1]*N[2]=abs(N[1])abs(N[2])cos(theta)}}}
{{{abs(N[1])=sqrt(3^2+(-1)^2+1^2)=sqrt(9+1+1)=sqrt(11)}}}
{{{abs(N[2])=sqrt(1^2+2^2+2^2)=sqrt(1+4+4)=sqrt(9)=3}}}
{{{N[1]*N[2]=3(1)+(-1)(2)+1(2)=3-2+2=3}}}
So then,
{{{3=sqrt(11)*3*cos(theta)}}}
{{{cos(theta)=1/sqrt(11)}}}
{{{theta=72.5}}}{{{degrees}}}