5/6+3/8=20/24+9/24=29/24=1 5/24
5/9-5/12=29/36-15/36=14/36
7/12-7/20=35/60-21/60=14/60
5/42+10/63=15/126+20/126=35/126
5/24 +7/60=25/120+14/120=39/120
3/4*5/7=15/28
1/8*12=1/8*12/1=12/8
Г) a1=-5, d=-7, n=14;
S14=(2a1+13d)*14/2=(2*(-5)+13*(-7))*7=(-10-91)*7=-101*7=-707.
Ответ: -707.
д) a1=8, d=-3, n=40;
S40=(2a1+39d)*40/2=(2*8+39*(-3))*20=(16-117)*20=-101*20=-2020.
Ответ: -2020.
Вроде так)
cos^2x-3sinxcosx=-1
cos^2x-3sincos+sin^2+cos^2=0
2cos^2+sin^2-3sincos=0
2+tg^2-3tg=0
D=1
X1,2= 2;1
tgx=2 tgx=1
x=arctg2+pk x=p/4+pk
x=p-arctg2+pk