50)d=5a+2b+a-3b=6a-b

|d|^2=(6a-b)^2=36a^2-12ab+b^2=36*8-12*√8*3*cos45+9=297-72=225

|d|=15

51)|p+q|*|p-q|*cosx=8*4+4*1=36

p+q=8i+4j+4i+j=12i+5j

|p+q|^2=12^2+5^2=144+25=13^2

|p+q|=13

p-q=8i+4j-4i-j=4i+3j

|p-q|^2=4^2+3^2=5^2

|p-q|=5

|p+q|*|p-q|*cosx=13*5*cosx=36

cosx=36/65≈0.554

arccos(0.554)≈56 градусов