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