C/C++の宿題なら俺にやらせろ！

main(){
　printf("f(0.8) = %d\n", x * ( x * ( x * (1.1 * x + 2.6 ) + 0.5 ) - 2.0 ) + 1.0 );
　printf("f'(0.8) = %d\n", x * ( x * (4.4 * x + 7.8 ) + 1.0 ) - 2.0 );
}

うは、つっこまれる前に修正
main(){
　printf("f(0.8) = %f\n", x * ( x * ( x * (1.1 * x + 2.6 ) + 0.5 ) - 2.0 ) + 1.0 );
　printf("f'(0.8) = %f\n", x * ( x * (4.4 * x + 7.8 ) + 1.0 ) - 2.0 );
}

main(){
　double x = 0.8;
　printf("f(0.8) = %f\n", x * ( x * ( x * (1.1 * x + 2.6 ) + 0.5 ) - 2.0 ) + 1.0 );
　printf("f'(0.8) = %f\n", x * ( x * (4.4 * x + 7.8 ) + 1.0 ) - 2.0 );
}

x = 0.8;

fx = 1.1;
fx = fx * x + 2.6;
fx = fx * x + 0.5;
fx = fx * x - 2.0;
fx = fx * x + 1.0;

printf("f(%f) = %f\n", x, fx);

dydx = 4.4;
dydx = dydx * x + 7.8;
dydx = dydx * x + 1.0;
dydx = dydx * x - 2.0;

printf("f'(%f) = %f\n", x, dydx);