По билинейности скалярного произведения
(4a + 8b)(10a – 3b) = 40a^2 + 80ab - 12ab - 24b^2 = 40a^2 + 68ab - 24b^2.
a^2 = |a|^2 = 4
ab = |a|*|b|*cos(60) = 1/2*|a|*|b| = 2
b^2 = |b|^2 = 4
Итого: 40*4 + 68*2 - 24*4 = 200
-π/4+2πk≤x/4 -1≤5π/4+2πk
-π/4 +1+2πk≤x/4≤5π/4 +1+2πk
-π+4+8πk≤x≤5π +4+8πk,k∈z
4/25+15/4=(4*4)+(15*25)/100=16+375/100=391/100=3.91