3) 1, 2(b + 1, 2) + (0, 5 - b)(b + 0, 5) - (b + 1, 3)(1, 3 - b) npи b = 5/6​

1,2(b+1,2)+(0,5-b)(b+0,5)-(b+1,3)(1,3-b)=

=1,2b+1,44+(0,5-b)(0,5+b)-(1,3+b)(1,3-b)=

=1,2b+1,44+0,5b^2-b^2-(1,3^2-b^2)=

=1,2b+1,44+0,5^2-b^2-((13/10)^2-b^2)=

=1,2b+1,44+0,5^2-b^2-(13/10)^2+b^2=

=1,2b+36/25+(1/2)^2-169/100=

=1,2b+36/25+1/4-169/100=

=1,2b+0=1,2b

при b=5/6

1,2b=1,2*(-5/6)=-1