A)18-6x-5+7x=4
x=4-18+5
x=-9
b)0,6x-1-0,3x-0,3=-0,7
0,3x=-0,7+1+0,3
x=2
Cos(105) = cos (60+45)
cos (75) = cos (30+45)
cos (60+45) = 1/2×√2/2 –√3/2×√2/2
√2/2(1/2–√3/2) =(1–√3)/2√2
cos(30+45) = √3/2×√2/2 –1/2×√2/2
√2/2(√3/2-1/2) = (√3–1)/2×√2/2= (√3–1)/2√2
(1–√3) – (1–√3) = 0
b1=-7 bn+1=3bn bn+1/bn=3 q=3
s5=b1*(qⁿ-1)/(q-1)
s5=-7*(3⁵-1)/2=-7*242/2=-847