1) sin105= sin(60+45)= sin60*cos45+cos60*sin45= √3/2 * √2/2 + 1/2 * √2/2= √6/4 + √2/4= (√6+√2)/4
2) sin(180-a)+cos(90+a)= sina+(- sina)= sina -sina =0
3) cos 40/sin40 * sin140/cos140 = (cos40*sin140)/(sin40*cos140)= 1/2 [sin(40+140)-sin(40-140)]/ 1/2 [sin(40+140)+sin(40-140)] = (sin180+sin100)/(sin180-sin100) = sin100/ (-sin100)= -1
5(49х-285)=81-6х
245х-1425=81-6х
245х+6х=81+1425
251х=1506
х=1506:251
х=6
81 = 9/(2x+3)
9 = 1/(2x+3)
9(2x+3) = 1
18x + 27 = 1
18x = -26
9x = -13
x = -13/9