А - 2 , Б - 1 , В - 4
.........................................................................................
B4=54, b3 = 54:3=18, b2=18:3=6, b1=6:3=2
2+6+18=26
10a-15b/16a-24b
5(2a-3b)/8(2a-3b)=5/8
Tg²x+ctg²x+2-1/cos²x=tg²x+ctg²x+2-(1+tg²x)=tg²x+ctg²x+2-1-tg²x=1+ctg²x=1/sin²x